java - Android 中如何将用户输入从一个 Activity 传递到另一个 Activity

Question

我正在创建一个基本的转换应用程序，它可以将用户输入的公里节转换为单位，反之亦然。目前，它在用户最初输入图形的位置显示结果。我想知道如何在新 Activity 中显示结果？目前，正如我所说，我只能设法让它出现在用户最初输入数字的位置，一秒钟后弹出第二个 Activity 。我在这里缺少什么？请看我的代码，谢谢:

MainActivity.java

package winfield.joe.wind.v1;

import android.os.Bundle;
import android.app.Activity;
import android.content.Intent;
import android.view.View;
import android.widget.EditText;
import android.widget.RadioButton;
import android.widget.Toast;

public class MainActivity extends Activity {

    // public var
    private EditText text;

    // default func
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Toast.makeText(this, "onCreate!", Toast.LENGTH_LONG).show();
        setContentView(R.layout.activity_main);
        // findViewById = Finds a view that was identified by the id attribute
        // from the XML that was processed in onCreate(Bundle).
        // (EditText) = typecast
        text = (EditText) findViewById(R.id.userInput);
    }

    //Will be executed by clicking on the calculate button because we assigned "calc" to the "onClick" Property

    public void calc(View view) {

        RadioButton toKilometers = (RadioButton) findViewById(R.id.toKilometers);
        RadioButton toKnots = (RadioButton) findViewById(R.id.toKnots);

        if (text.getText().length() == 0) {
            // if the text field is empty show the message "enter a valid number" via toast message
            Toast.makeText(this, "enter a valid number", Toast.LENGTH_LONG).show();
        } else {

            int result = R.string.userInput;
            Intent i = new Intent(MainActivity.this, SecondActivity.class);
            putExtra("userInput", result); 
            startActivity(i);

            // parse input Value from Text Field
            double inputValue = Double.parseDouble(text.getText().toString());
            // convert to...
            if (toKilometers.isChecked()) {
                text.setText(String.valueOf(convertToKM(inputValue)));
                // uncheck "to km" Button
                toKilometers.setChecked(false);
                // check "to knots" Button
                toKnots.setChecked(true);
            } else { /* if toKnots button isChecked() */
                text.setText(String.valueOf(convertToKnots(inputValue)));
                // uncheck "to knots" Button
                toKnots.setChecked(false);
                // check "to km" Button
                toKilometers.setChecked(true);
            }
        }

    }


    private void putExtra(String string, int result) {
        // TODO Auto-generated method stub
    }

    private double convertToKM(double inputValue) {
        // convert knots to km
        return (inputValue * 1.8);
    }

    private double convertToKnots(double inputValue) {
        // convert km to knots
        return (inputValue * 0.539956803);
    }

}

SecondActivity.java

package winfield.joe.wind.v1;

import android.app.Activity;
import android.app.AlertDialog;
import android.content.DialogInterface;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;

public class SecondActivity extends Activity {

    Bundle extras = getIntent().getExtras();
    int result = extras.getInt("results"); 

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

    //onClick GoBack method assigned to the Go Back? button which returns the user to main activity from the second activity
    public void GoBack(View view) {

        AlertDialog.Builder builder = new AlertDialog.Builder(this);
        builder.setTitle("Are you sure?");

            // set dialog message
            builder .setCancelable(false)

                    .setPositiveButton("Convert Again",new DialogInterface.OnClickListener() {
                        public void onClick(DialogInterface dialog,int id) {
                            //You return to Main Activity  
                            Intent intent = new Intent(SecondActivity.this, MainActivity.class);
                            startActivity(intent);
                        }
                    })

                    .setNeutralButton("Back to Home",new DialogInterface.OnClickListener() {
                        public void onClick(DialogInterface dialog,int id) {
                            //You return to the home page
                            Intent intent = new Intent(Intent.ACTION_MAIN);
                            intent.addCategory(Intent.CATEGORY_HOME);
                            startActivity(intent);
                        }
                    })

                    .setNegativeButton("View Results",new DialogInterface.OnClickListener() {
                        public void onClick(DialogInterface dialog,int id) {
                            // if this button is clicked, just close the dialog box and do nothing
                            dialog.cancel();
                        }
                    });

        AlertDialog alert = builder.create();
        alert.show();

    }

}

activity_main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/LinearLayout1"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:layout_alignParentBottom="true"
    android:layout_alignParentEnd="true"
    android:background="@color/bgColor"
    android:orientation="vertical">

    <!--TITLE-->

    <TextView
        android:id="@+id/titleMain"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:layout_marginTop="22dp"
        android:gravity="center"
        android:text="@string/titleMain"
        android:textColor="@color/textColor"
        android:textColorHint="@color/textColor"
        android:textColorLink="#ffffff"
        android:textSize="30sp" />

    <!--USER-INPUT-->

    <EditText
        android:id="@+id/userInput"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginLeft="20dp"
        android:layout_marginRight="20dp"
        android:ems="10"
        android:hint="@+string/userInput"
        android:inputType="numberDecimal"
        android:paddingEnd="40dp"
        android:paddingStart="20dp"
        android:paddingTop="30dp" >

        <requestFocus />
    </EditText>

      <!--TWO RADIO BUTTONS-->
      <LinearLayout

          android:layout_width="match_parent"
          android:layout_height="wrap_content"
          android:paddingLeft="10dp"
          android:paddingRight="10dp">

          <!--toKNOTS-->

          <RadioButton
              android:id="@+id/toKnots"
              android:layout_width="0dp"
              android:layout_height="wrap_content"
              android:layout_gravity="center"
              android:layout_marginStart="10dp"
              android:layout_marginTop="10dp"
              android:layout_weight="0.04"
              android:checked="true"
              android:text="@string/toKnots" />

          <!--toKM-->

          <RadioButton
              android:id="@+id/toKilometers"
              android:layout_width="wrap_content"
              android:layout_height="wrap_content"
              android:layout_gravity="center"
              android:layout_marginEnd="20dp"
              android:layout_marginTop="10dp"
              android:text="@string/toKilometers" />

      </LinearLayout>

      <!--CALCULATE-->

      <Button
          android:id="@+id/valueCalc"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:layout_gravity="center"
          android:layout_marginEnd="10dp"
          android:layout_marginStart="10dp"
          android:layout_marginTop="30dp"
          android:onClick="calc"
          android:text="@string/valueCalc" />

</LinearLayout>

activity_second.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/LinearLayout1"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:layout_alignParentBottom="true"
    android:layout_alignParentEnd="true"
    android:background="@color/bgColor"
    android:orientation="vertical"
    tools:context="${relativePackage}.${activityClass}" >

    <!-- TITLE -->

    <TextView
        android:id="@+id/titleResults"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:layout_marginTop="24dp"
        android:gravity="center"
        android:text="@string/titleResults"
        android:textColor="@color/textColor"
        android:textColorHint="@color/textColor"
        android:textColorLink="#ffffff"
        android:textSize="25sp" />

    <EditText
        android:id="@+id/results"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:layout_margin="20dp"
        android:layout_marginTop="30dp"
        android:ems="10"
        android:hint="@string/results"
        android:inputType="numberDecimal"
        android:textColorHint="@color/textColor" >

        <requestFocus />
    </EditText>

    <Button
        android:id="@+id/GoBack"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:layout_marginTop="40dp"
        android:text="@string/GoBack"
        android:onClick="GoBack" />

</LinearLayout>

LOGCAT

03-09 12:27:35.692: E/AndroidRuntime(2911): java.lang.RuntimeException: Unable to start activity ComponentInfo{winfield.joe.wind.v1/winfield.joe.wind.v1.SecondActivity}: java.lang.NullPointerException
03-09 12:27:35.692: E/AndroidRuntime(2911):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
03-09 12:27:45.262: E/AndroidRuntime(2934): java.lang.RuntimeException: Unable to start activity ComponentInfo{winfield.joe.wind.v1/winfield.joe.wind.v1.SecondActivity}: java.lang.NullPointerException
03-09 12:27:45.262: E/AndroidRuntime(2934):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
03-09 12:29:52.172: E/AndroidRuntime(2989): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{winfield.joe.wind.v1/winfield.joe.wind.v1.SecondActivity}: java.lang.NullPointerException
03-09 12:29:52.172: E/AndroidRuntime(2989):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
03-09 12:32:29.322: E/AndroidRuntime(3056): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{winfield.joe.wind.v1/winfield.joe.wind.v1.SecondActivity}: java.lang.NullPointerException
03-09 12:32:29.322: E/AndroidRuntime(3056):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
03-09 12:36:29.872: E/AndroidRuntime(3112): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{winfield.joe.wind.v1/winfield.joe.wind.v1.SecondActivity}: java.lang.NullPointerException
03-09 12:36:29.872: E/AndroidRuntime(3112):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
03-09 12:44:25.022: E/AndroidRuntime(3172): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{winfield.joe.wind.v1/winfield.joe.wind.v1.SecondActivity}: java.lang.NullPointerException
03-09 12:44:25.022: E/AndroidRuntime(3172):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
03-09 12:53:38.802: E/AndroidRuntime(3242): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{winfield.joe.wind.v1/winfield.joe.wind.v1.SecondActivity}: java.lang.NullPointerException
03-09 12:53:38.802: E/AndroidRuntime(3242):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
03-09 13:19:58.122: E/AndroidRuntime(3299): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{winfield.joe.wind.v1/winfield.joe.wind.v1.SecondActivity}: java.lang.NullPointerException
03-09 13:19:58.122: E/AndroidRuntime(3299):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)

Answer 1

如果您想将任何内容从一个 Activity 传递到另一个 Activity ，您可以通过使用 putExtra 来实现，它会将数据从一个 Activity 发送到另一个 Activity ，并在另一个 Activity 中获取该数据通过使用getExtras。

发送数据，

String str = "value";
Intent launchResult = new Intent(MainActivity.this, SecondActivity.class);
putExtra("key", str);
startActivity(launchResult);

要在另一个 Activity 中接收数据，

Bundle extras = getIntent().getExtras();
String str = extras.getStr("key");

编辑:

在我的回答中，key 是在发送方 Activity 和接收方 Activity 中必须保持相同的任何唯一值 .

您收到错误是因为您传递了一个字符串值，而您必须传递 key ，而接收器 Activity 中的 key 也不匹配。

查看下面的更改，它应该有效

MainActivity.class

int result = R.string.userInput;
Intent i = new Intent(MainActivity.this, SecondActivity.class);
putExtra("userInput", result); 
startActivity(i);

SecondActivity.class

Bundle extras = getIntent().getExtras();
int result = extras.getInt("userInput");