java - 如何将 BigDecimal 正确显示为字符串

Question

我正在尝试学习java以进入android编程
对于我的练习，我正在尝试编写一个带有基本算术运算和小数点按钮的计算器程序
我尝试使用原始数字类型(例如 int 和 double)来执行此操作，但是如果我使用 double 作为变量类型，则诸如 2.111 之类的数字将显示为 2.11099999，但除此之外，它似乎工作正常( my question about this is given here )< br/>正如我的另一个问题的答案中所建议的，我修改了程序以使用 BigDecimal 数据类型而不是 double，但是现在当我单击 1 按钮时，尾数显示为 1.0，而不是像之前的程序中那样仅显示 1用过双
我怎样才能解决这个问题？

更新尾数的代码:

public void updateMantissa(){

    // mantissa_valueBD = BigDecimal.valueOf(mantissa_value);

    if(mantissa_value == BigDecimal.ZERO){
        mantissa_str = "0";
    }
    /*
    else if(ceil(mantissa_value) == mantissa_value){
        mantissa_str = String.valueOf(mantissa_valueBD);
        mantissa_str = mantissa_str.substring(0, mantissa_str.length() - 2 ); 
    }

    else{
        mantissa_str = String.valueOf(mantissa_valueBD);
        mantissa_str = mantissa_str.substring(0, Math.min(mantissa_str.length(), 10));
    }
    */

    // /*
    else if(mantissa_value.setScale(0, RoundingMode.CEILING)== mantissa_value){
        // mantissa_str = String.valueOf(mantissa_value);
        mantissa_str = mantissa_value.toString();
        mantissa_str = mantissa_str.substring(0, mantissa_str.length() - 4 ); 
    }

    else{
        mantissa_str = String.valueOf(mantissa_value);
        mantissa_str = mantissa_str.substring(0, Math.min(mantissa_str.length(), 10));
    }
    // */
    TextView thisText = (TextView) findViewById(R.id.mantissa);
    thisText.setText(mantissa_str);
}

完整代码(MainActivity 类):

package com.example.calculator;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;
import android.widget.TextView;
import static java.lang.Math.pow;
// import static java.lang.Math.ceil;
import java.math.BigDecimal;
import java.math.RoundingMode;

public class MainActivity extends Activity {
// private double mantissa_value = 0;
// private BigDecimal mantissa_value = new BigDecimal(0);
// private BigDecimal opr1BD, opr2BD, resBD, mantissa_valueBD;
/*
private double opr1 = 0;
private double opr2 = 0;
private double res = 0;
// */

BigDecimal opr1 = BigDecimal.ZERO;
BigDecimal opr2 = BigDecimal.ZERO;
BigDecimal res = BigDecimal.ZERO;
BigDecimal tempBD = BigDecimal.ZERO;
BigDecimal mantissa_value = BigDecimal.ZERO;

private enum OprTypes {NONE, PLUS, MINUS, MULT, DIV}
OprTypes oprLatest = OprTypes.NONE;
private boolean oprPlusClicked = false;
private boolean oprMinusClicked = false;
private boolean oprMultClicked = false;
private boolean oprDivClicked = false;
private boolean decimalClicked = false;
private int numOfClicksAfterDecimal = 0;
private String mantissa_str;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    updateMantissa();
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

public void updateMantissa(){

    // mantissa_valueBD = BigDecimal.valueOf(mantissa_value);

    if(mantissa_value == BigDecimal.ZERO){
        mantissa_str = "0";
    }
    /*
    else if(ceil(mantissa_value) == mantissa_value){
        mantissa_str = String.valueOf(mantissa_valueBD);
        mantissa_str = mantissa_str.substring(0, mantissa_str.length() - 2 ); 
    }

    else{
        mantissa_str = String.valueOf(mantissa_valueBD);
        mantissa_str = mantissa_str.substring(0, Math.min(mantissa_str.length(), 10));
    }
    */

    // /*
    else if(mantissa_value.setScale(0, RoundingMode.CEILING)== mantissa_value){
        // mantissa_str = String.valueOf(mantissa_value);
        mantissa_str = mantissa_value.toString();
        mantissa_str = mantissa_str.substring(0, mantissa_str.length() - 4 ); 
    }

    else{
        mantissa_str = String.valueOf(mantissa_value);
        mantissa_str = mantissa_str.substring(0, Math.min(mantissa_str.length(), 10));
    }
    // */
    TextView thisText = (TextView) findViewById(R.id.mantissa);
    thisText.setText(mantissa_str);
}

public void onClkBn0(View view){
    TextView thisText = (TextView) findViewById(R.id.mantissa);
    if(decimalClicked == false){
        tempBD = mantissa_value.multiply(BigDecimal.valueOf(10.0));
        mantissa_value = mantissa_value.add(BigDecimal.valueOf(0.0));
        updateMantissa();
    }
    else{
        if(mantissa_value == BigDecimal.ZERO){
            if(numOfClicksAfterDecimal == 1){
                mantissa_str = mantissa_str + ".0";
            }
            else{
                mantissa_str = mantissa_str + "0";
            }
        }

        else if(mantissa_value.setScale(0, RoundingMode.CEILING)== mantissa_value){
            if(numOfClicksAfterDecimal == 1){
                mantissa_str = mantissa_str + ".0";
            }
            else{
                mantissa_str = mantissa_str + "0";
            }
        }

        // mantissa_value = mantissa_value + pow(10, -numOfClicksAfterDecimal)*0;
        tempBD = BigDecimal.valueOf(pow(10, -numOfClicksAfterDecimal)*0);
        mantissa_value = mantissa_value.add(tempBD);
        numOfClicksAfterDecimal++;
        thisText.setText(mantissa_str);
    }
}

public void onClkBn1(View view){
    if(decimalClicked == false){
        // mantissa_value = mantissa_value*10+1;
        tempBD = mantissa_value.multiply(BigDecimal.valueOf(10.0));
        mantissa_value = tempBD.add(BigDecimal.valueOf(1.0));
    }

    else{
        // mantissa_value = mantissa_value + pow(10, -numOfClicksAfterDecimal)*1;
        tempBD = BigDecimal.valueOf(pow(10, -numOfClicksAfterDecimal)*1);
        mantissa_value = mantissa_value.add(tempBD);
        numOfClicksAfterDecimal++;
    }
    updateMantissa();
}

public void onClkBn2(View view){
    if(decimalClicked == false){
        // mantissa_value = mantissa_value*10+2;
        tempBD = mantissa_value.multiply(BigDecimal.valueOf(10.0));
        mantissa_value = tempBD.add(BigDecimal.valueOf(2.0));
    }
    else{
        // mantissa_value = mantissa_value + pow(10, -numOfClicksAfterDecimal)*2;
        tempBD = BigDecimal.valueOf(pow(10, -numOfClicksAfterDecimal)*2);
        mantissa_value = mantissa_value.add(tempBD);
        numOfClicksAfterDecimal++;
    }
    updateMantissa();
}

public void onClkBn3(View view){
    if(decimalClicked == false){
        // mantissa_value = mantissa_value*10+3;
        tempBD = mantissa_value.multiply(BigDecimal.valueOf(10.0));
        mantissa_value = tempBD.add(BigDecimal.valueOf(3.0));
    }
    else{
        // mantissa_value = mantissa_value + pow(10, -numOfClicksAfterDecimal)*3;
        tempBD = BigDecimal.valueOf(pow(10, -numOfClicksAfterDecimal)*3);
        mantissa_value = mantissa_value.add(tempBD);
        numOfClicksAfterDecimal++;
    }
    updateMantissa();
}

public void onClkBn4(View view){
    if(decimalClicked == false){
        // mantissa_value = mantissa_value*10+4;
        tempBD = mantissa_value.multiply(BigDecimal.valueOf(10.0));
        mantissa_value = tempBD.add(BigDecimal.valueOf(4.0));
    }
    else{
        // mantissa_value = mantissa_value + pow(10, -numOfClicksAfterDecimal)*4;
        tempBD = BigDecimal.valueOf(pow(10, -numOfClicksAfterDecimal)*4);
        mantissa_value = mantissa_value.add(tempBD);
        numOfClicksAfterDecimal++;
    }
    updateMantissa();
}

public void onClkBn5(View view){
    if(decimalClicked == false){
        // mantissa_value = mantissa_value*10+5;
        tempBD = mantissa_value.multiply(BigDecimal.valueOf(10.0));
        mantissa_value = tempBD.add(BigDecimal.valueOf(5.0));
    }
    else{
        // mantissa_value = mantissa_value + pow(10, -numOfClicksAfterDecimal)*5;
        tempBD = BigDecimal.valueOf(pow(10, -numOfClicksAfterDecimal)*5);
        mantissa_value = mantissa_value.add(tempBD);
        numOfClicksAfterDecimal++;
    }
    updateMantissa();
}

public void onClkBn6(View view){
    if(decimalClicked == false){
        // mantissa_value = mantissa_value*10+6;
        tempBD = mantissa_value.multiply(BigDecimal.valueOf(10.0));
        mantissa_value = tempBD.add(BigDecimal.valueOf(6.0));
    }
    else{
        // mantissa_value = mantissa_value + pow(10, -numOfClicksAfterDecimal)*6;
        tempBD = BigDecimal.valueOf(pow(10, -numOfClicksAfterDecimal)*6);
        mantissa_value = mantissa_value.add(tempBD);
        numOfClicksAfterDecimal++;
    }
    updateMantissa();
}

public void onClkBn7(View view){
    if(decimalClicked == false){
        // mantissa_value = mantissa_value*10+7;
        tempBD = mantissa_value.multiply(BigDecimal.valueOf(10.0));
        mantissa_value = tempBD.add(BigDecimal.valueOf(7.0));
    }
    else{
        // mantissa_value = mantissa_value + pow(10, -numOfClicksAfterDecimal)*7;
        tempBD = BigDecimal.valueOf(pow(10, -numOfClicksAfterDecimal)*7);
        mantissa_value = mantissa_value.add(tempBD);
        numOfClicksAfterDecimal++;
    }
    updateMantissa();
}

public void onClkBn8(View view){
    if(decimalClicked == false){
        // mantissa_value = mantissa_value*10+8;
        tempBD = mantissa_value.multiply(BigDecimal.valueOf(10.0));
        mantissa_value = tempBD.add(BigDecimal.valueOf(8.0));
    }
    else{
        // mantissa_value = mantissa_value + pow(10, -numOfClicksAfterDecimal)*8;
        tempBD = BigDecimal.valueOf(pow(10, -numOfClicksAfterDecimal)*8);
        mantissa_value = mantissa_value.add(tempBD);
        numOfClicksAfterDecimal++;
    }
    updateMantissa();
}

public void onClkBn9(View view){
    if(decimalClicked == false){
        // mantissa_value = mantissa_value*10+9;
        tempBD = mantissa_value.multiply(BigDecimal.valueOf(10.0));
        mantissa_value = tempBD.add(BigDecimal.valueOf(9.0));
    }
    else{
        // mantissa_value = mantissa_value + pow(10, -numOfClicksAfterDecimal)*9;
        tempBD = BigDecimal.valueOf(pow(10, -numOfClicksAfterDecimal)*9);
        mantissa_value = mantissa_value.add(tempBD);
        numOfClicksAfterDecimal++;
    }
    updateMantissa();
}

public void onClkBnDot(View view){
    decimalClicked = true;
}

public void onClkBnCan(View view){
    mantissa_value = BigDecimal.ZERO;
    opr1 = BigDecimal.ZERO;
    opr2 = BigDecimal.ZERO;
    res = BigDecimal.ZERO;
    /*
    opr1 = 0;
    opr2 = 0;
    res = 0;
    // */
    oprLatest = OprTypes.NONE;
    oprPlusClicked = false;
    oprMinusClicked = false;
    oprMultClicked = false;
    oprDivClicked = false;
    decimalClicked = false;
    numOfClicksAfterDecimal = 1;
}


public void onClkBnPlus(View view){
    if(oprPlusClicked == false){
        oprPlusClicked = true;
        opr1 = mantissa_value;
        // mantissa_value = 0;
        mantissa_value = BigDecimal.ZERO;
    }
    else{
        // this should mean opr1 already has some value in it
        // so add the current mantissa value to opr1
        // opr1 = opr1 + mantissa_value;
        opr1 = opr1.add(mantissa_value);
        mantissa_value = BigDecimal.ZERO;
    }
    oprLatest = OprTypes.PLUS;
}


public void onClkBnMinus(View view){
    if(oprMinusClicked == false){
        oprMinusClicked = true;
        opr1 = mantissa_value;
        mantissa_value = BigDecimal.ZERO;
    }
    else{
        // this should mean opr1 already has some value in it
        // so add the current mantissa value to opr1
        // opr1 = opr1 - mantissa_value;
        opr1 = opr1.subtract(mantissa_value);
        mantissa_value = BigDecimal.ZERO;
    }
    oprLatest = OprTypes.MINUS;
}

public void onClkBnMult(View view){
    if(oprMultClicked == false){
        oprMultClicked = true;
        opr1 = mantissa_value;
        mantissa_value = BigDecimal.ZERO;
    }
    else{
        // this should mean opr1 already has some value in it
        // so add the current mantissa value to opr1
        // opr1 = opr1 * mantissa_value;
        opr1 = opr1.multiply(mantissa_value);
        mantissa_value = BigDecimal.ZERO;
    }
    oprLatest = OprTypes.MULT;
}

public void onClkBnDiv(View view){
    if(oprDivClicked == false){
        oprDivClicked = true;
        opr1 = mantissa_value;
        mantissa_value = BigDecimal.ZERO;
    }
    else{
        // this should mean opr1 already has some value in it
        // so add the current mantissa value to opr1
        // opr1 = opr1 / mantissa_value;
        opr1 = opr1.divide(mantissa_value);
        mantissa_value = BigDecimal.ZERO;
    }
    oprLatest = OprTypes.DIV;
}


public void onClkBnRes(View view){
    if(oprPlusClicked == true || oprMinusClicked == true || oprMultClicked == true || oprDivClicked == true){
        switch(oprLatest){
        case PLUS:
            // mantissa_value = opr1 + mantissa_value;
            mantissa_value = opr1.add(mantissa_value);
            break;
        case MINUS:
            // mantissa_value = opr1 - mantissa_value;
            mantissa_value = opr1.subtract(mantissa_value);
            break;
        case MULT:
            // mantissa_value = opr1 * mantissa_value;
            mantissa_value = opr1.multiply(mantissa_value);
            break;
        case DIV:
            // mantissa_value = opr1 / mantissa_value;
            mantissa_value = opr1.divide(mantissa_value);
            break;
        case NONE:
            break;
        }
    }
    oprLatest = OprTypes.NONE;
    oprPlusClicked = false;
    oprMinusClicked = false;
    oprMultClicked = false;
    oprDivClicked = false;
    updateMantissa();
}
 }

------编辑------
如果我做这样的事情，

public static void main(String[] args){
    // double a;
    a = 2.111;
    System.out.println(a);
}

它准确地显示 2.111，而不是 2.1109999。这是为什么？

------ 编辑 2 ------

我对程序的逻辑进行了更改，以便它现在可以在字符串上执行与显示相关的任何操作，同时使用 Double.parseDouble() 函数将尾数字符串值转换为 double 以执行与显示相关的任何操作计算。如果有人想评论的话，更新后的代码如下所示

package com.example.calculator;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;
import android.widget.TextView;
import static java.lang.Math.pow;
import static java.lang.Math.ceil;
// import java.math.BigDecimal;
import java.text.NumberFormat;

public class MainActivity extends Activity {

     private double mantissa_value = 0;
    // private BigDecimal mantissa_value = new BigDecimal(0);
    // private BigDecimal opr1BD, opr2BD, resBD, mantissa_valueBD;
    // /*
    private double opr1 = 0;
    // */
    NumberFormat number =  NumberFormat.getNumberInstance();

    private enum OprTypes {NONE, PLUS, MINUS, MULT, DIV}
    OprTypes oprLatest = OprTypes.NONE;
    private boolean oprPlusClicked = false;
    private boolean oprMinusClicked = false;
    private boolean oprMultClicked = false;
    private boolean oprDivClicked = false;
    private boolean decimalClicked = false;
    private boolean resetMantissa = true;
    private String mantissa_str;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        // updateMantissa();
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    public void updateMantissa(){
        // This is the version that should get called from the num pad buttons, 0 - 9
        updateMantissa(true);
    }
    public void updateMantissa(boolean frmNumPad){
        // this is the version that should get called from other buttons besides the num pad buttons
        if(frmNumPad != true){
            long mantissa_val_int_part = (long)mantissa_value;
            if(mantissa_value == (double)mantissa_val_int_part){
                mantissa_str = mantissa_str.substring(0, mantissa_str.length() - 2 );
            }
            else if (mantissa_str.length() > 10){
                mantissa_str = mantissa_str.substring(0, Math.min(mantissa_str.length(), 10));
            }

        }


        TextView thisText = (TextView) findViewById(R.id.mantissa);
        thisText.setText(mantissa_str);
    }

    public void onClkBn0(View view){
        if (resetMantissa == true){
            mantissa_str = "";
            resetMantissa = false;
        }

        if(decimalClicked == false){
            if( !(Double.parseDouble(mantissa_str)==0.0) ) 
                mantissa_str = mantissa_str + "0";
        }
        else{
            mantissa_str = mantissa_str + "0";
        }
        TextView thisText = (TextView) findViewById(R.id.mantissa);
        thisText.setText(mantissa_str);
    }

    public void onClkBn1(View view){
        if (resetMantissa == true){
            mantissa_str = "";
            resetMantissa = false;
        }
        mantissa_str = mantissa_str + "1";
        mantissa_value = Double.parseDouble(mantissa_str);
        updateMantissa();
    }

    public void onClkBn2(View view){
        if (resetMantissa == true){
            mantissa_str = "";
            resetMantissa = false;
        }
        mantissa_str = mantissa_str + "2";
        mantissa_value = Double.parseDouble(mantissa_str);
        updateMantissa();
    }

    public void onClkBn3(View view){
        if (resetMantissa == true){
            mantissa_str = "";
            resetMantissa = false;
        }
        mantissa_str = mantissa_str + "3";
        mantissa_value = Double.parseDouble(mantissa_str);
        updateMantissa();
    }

    public void onClkBn4(View view){
        if (resetMantissa == true){
            mantissa_str = "";
            resetMantissa = false;
        }
        mantissa_str = mantissa_str + "4";
        mantissa_value = Double.parseDouble(mantissa_str);
        updateMantissa();
    }

    public void onClkBn5(View view){
        if (resetMantissa == true){
            mantissa_str = "";
            resetMantissa = false;
        }
        mantissa_str = mantissa_str + "5";
        mantissa_value = Double.parseDouble(mantissa_str);
        updateMantissa();
    }

    public void onClkBn6(View view){
        if (resetMantissa == true){
            mantissa_str = "";
            resetMantissa = false;
        }
        mantissa_str = mantissa_str + "6";
        mantissa_value = Double.parseDouble(mantissa_str);
        updateMantissa();
    }

    public void onClkBn7(View view){
        if (resetMantissa == true){
            mantissa_str = "";
            resetMantissa = false;
        }
        mantissa_str = mantissa_str + "7";
        mantissa_value = Double.parseDouble(mantissa_str);
        updateMantissa();
    }

    public void onClkBn8(View view){
        if (resetMantissa == true){
            mantissa_str = "";
            resetMantissa = false;
        }
        mantissa_str = mantissa_str + "8";
        mantissa_value = Double.parseDouble(mantissa_str);
        updateMantissa();
    }

    public void onClkBn9(View view){
        if (resetMantissa == true){
            mantissa_str = "";
            resetMantissa = false;
        }
        mantissa_str = mantissa_str + "9";
        mantissa_value = Double.parseDouble(mantissa_str);
        updateMantissa();
    }

    public void onClkBnDot(View view){
        if(decimalClicked == false){
            decimalClicked = true;
            mantissa_str = mantissa_str + ".";
        }
    }

    public void onClkBnCan(View view){
        mantissa_value = 0;
        // /*
        opr1 = 0;
        // */
        oprLatest = OprTypes.NONE;
        oprPlusClicked = false;
        oprMinusClicked = false;
        oprMultClicked = false;
        oprDivClicked = false;
        decimalClicked = false;
        mantissa_value = 0;
        opr1 = 0;
        resetMantissa = true;
        updateMantissa(false);
    }


    public void onClkBnPlus(View view){
        if(oprPlusClicked == false){
            oprPlusClicked = true;
            decimalClicked = false;
            opr1 = Double.parseDouble(mantissa_str);
            mantissa_value = 0;
        }
        else{
            // this should mean opr1 already has some value in it
            // so add the current mantissa value to opr1
            mantissa_value = Double.parseDouble(mantissa_str);
            opr1 = opr1 + mantissa_value;
            mantissa_value = opr1;
            decimalClicked = false;
            mantissa_str = Double.toString(mantissa_value);
            updateMantissa(false);
        }
        oprLatest = OprTypes.PLUS;
        resetMantissa = true;
    }


    public void onClkBnMinus(View view){
        if(oprMinusClicked == false){
            oprMinusClicked = true;
            decimalClicked = false;
            opr1 = Double.parseDouble(mantissa_str);
            mantissa_value = 0;
        }
        else{
            // this should mean opr1 already has some value in it
            // so add the current mantissa value to opr1
            mantissa_value = Double.parseDouble(mantissa_str);
            opr1 = opr1 - mantissa_value;
            mantissa_value = opr1;
            decimalClicked = false;
            mantissa_str = Double.toString(mantissa_value);
            updateMantissa(false);
        }
        oprLatest = OprTypes.MINUS;
        resetMantissa = true;
    }

    public void onClkBnMult(View view){
        if(oprMultClicked == false){
            oprMultClicked = true;
            decimalClicked = false;
            opr1 = Double.parseDouble(mantissa_str);
            mantissa_value = 0;
        }
        else{
            // this should mean opr1 already has some value in it
            // so add the current mantissa value to opr1
            mantissa_value = Double.parseDouble(mantissa_str);
            opr1 = opr1 * mantissa_value;
            mantissa_value = opr1;
            decimalClicked = false;
            mantissa_str = Double.toString(mantissa_value);
            updateMantissa(false);
        }
        oprLatest = OprTypes.MULT;
        resetMantissa = true;
    }

    public void onClkBnDiv(View view){
        if(oprDivClicked == false){
            oprDivClicked = true;
            decimalClicked = false;
            opr1 = Double.parseDouble(mantissa_str);
            mantissa_value = 0;
        }
        else{
            // this should mean opr1 already has some value in it
            // so add the current mantissa value to opr1
            mantissa_value = Double.parseDouble(mantissa_str);
            opr1 = opr1 / mantissa_value;
            mantissa_value = opr1;
            decimalClicked = false;
            mantissa_str = Double.toString(mantissa_value);
            updateMantissa(false);
        }
        oprLatest = OprTypes.DIV;
        resetMantissa = true;
    }


    public void onClkBnRes(View view){
        mantissa_value = Double.parseDouble(mantissa_str);
        if(oprPlusClicked == true || oprMinusClicked == true || oprMultClicked == true || oprDivClicked == true){
            switch(oprLatest){
            case PLUS:
                mantissa_value = opr1 + mantissa_value;
                break;
            case MINUS:
                mantissa_value = opr1 - mantissa_value;
                break;
            case MULT:
                mantissa_value = opr1 * mantissa_value;
                break;
            case DIV:
                mantissa_value = opr1 / mantissa_value;
                break;
            case NONE:
                break;
            }
        }
        oprLatest = OprTypes.NONE;
        oprPlusClicked = false;
        oprMinusClicked = false;
        oprMultClicked = false;
        oprDivClicked = false;
        decimalClicked = false;
        mantissa_str = Double.toString(mantissa_value);
        updateMantissa(false);
    }

    public void onClkBnOff(View view){
        finish();
    }

}

Answer 1

通过阅读您链接到的问题，尚不清楚您关心的是 float 精度问题还是 float 打印问题。最初问题中的人们建议支持 BigDecimal，但我感觉您最初的担忧与后者有关 - 即打印值 - 这就是您仍然面临的问题。

使用String.valueOf()打印 double 值几乎从来都不是一个好主意。幕后发生的事情是根据默认区域设置将浮点值转换为字符串。您应该自己执行转换，而不是让系统进行转换。

实现此目的的方法是使用 DecimalFormat 类。示例代码:

double mantissa_value = 2.11099999;

DecimalFormat df = new DecimalFormat("0.###");
// Un-comment the line below to take English locale into account when converting
//df.setDecimalFormatSymbols(new DecimalFormatSymbols(Locale.ENGLISH));

// will print '2.11099999' this is what you get in your original case
System.out.println( String.valueOf(mantissa_value) ); 

// will print '2.111' or '2,111' depending on your locale. if you uncomment the line above for English locale it will print '2.111'
System.out.println( df.format(mantissa_value) ); 

上述十进制格式将打印小数点后 1、2 或 3 位数字的 double 值。如果您需要可变精度，您可以在用户输入字符时在变量中保留一个计数器，然后使用该计数器生成具有所需十进制数字的 DecimalFormat 。对于 BigDecimal 情况可以采用相同的方法。我希望这有帮助