java - 如何在 Kotlin 计算器应用程序中重置变量

Question

所以我刚刚开始使用 Kotlin，所以我决定制作一个简单的计算器应用程序。计算器可以工作，但在尝试新的计算时，即使在清除屏幕后，它显然仍然可以使用以前的值。例如，2 + 2 将等于 4。如果尝试计算新数字，我只需按 CLS 按钮，屏幕就会变成空白。然而，问题是如果我想计算 2+3 这样的东西，它不会等于 5，而是等于 9。它会添加到之前的结果中。因此，即使在清除后，占位符仍保留先前计算的值。

我的清除屏幕监听器使用 clear() 从屏幕上删除数字，但不清除占位符。这是我的代码块

 val ClearButtonListener = View.OnClickListener { v ->
            val b = v as Button
            if (newNumber != null) {
                newNumber.getText().clear()
                result.getText().clear()
            }
        }

newNumber 和 result 是 EditText 小部件，并使用 Lateinit 进行声明。所以我的问题是如何将这些小部件重置回 null，而不是保留以前的值？

我的整个代码:

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.view.View
import android.widget.Button
import android.widget.EditText
import android.widget.TextView
import kotlinx.android.synthetic.main.activity_main.*
import java.lang.NumberFormatException

class MainActivity : AppCompatActivity() {
    private lateinit var result: EditText
    private lateinit var newNumber: EditText
    private val displayOperation by lazy(LazyThreadSafetyMode.NONE) { findViewById<TextView>(R.id.operation) }

    private var operand1: Double? = null
    private var pendingOperation = "="

    override fun onCreate(savedInstanceState: Bundle?) {

        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        result = findViewById(R.id.result)
        newNumber = findViewById(R.id.newNumber)
        //Data input buttons
        val button0: Button = findViewById(R.id.button0)
        val button1: Button = findViewById(R.id.button1)
        val button2: Button = findViewById(R.id.button2)
        val button3: Button = findViewById(R.id.button3)
        val button4: Button = findViewById(R.id.button4)
        val button5: Button = findViewById(R.id.button5)
        val button6: Button = findViewById(R.id.button6)
        val button7: Button = findViewById(R.id.button7)
        val button8: Button = findViewById(R.id.button8)
        val button9: Button = findViewById(R.id.button9)
        val buttonDot: Button = findViewById(R.id.buttonDecimal)

        val clearButton: Button = findViewById(R.id.clear)

        // operation buttons
        val Buttonequals: Button = findViewById(R.id.equal)
        val ButtonPlus: Button = findViewById(R.id.add)
        val ButtonMinus: Button = findViewById(R.id.Subtract)
        val ButtonMultiply: Button = findViewById(R.id.Multiply)
        val ButtonDivide: Button = findViewById(R.id.Divide)

        val listener = View.OnClickListener { v ->
            val b = v as Button
            newNumber.append(b.text)
        }

        //Clear button
        val ClearButtonListener = View.OnClickListener { v ->
            val b = v as Button
            if (newNumber != null) {
                newNumber.getText().clear()
                result.getText().clear()
            }
        }
        button0.setOnClickListener(listener)
        button1.setOnClickListener(listener)
        button2.setOnClickListener(listener)
        button3.setOnClickListener(listener)
        button4.setOnClickListener(listener)
        button5.setOnClickListener(listener)
        button6.setOnClickListener(listener)
        button7.setOnClickListener(listener)
        button8.setOnClickListener(listener)
        button9.setOnClickListener(listener)
        buttonDot.setOnClickListener(listener)

        clearButton.setOnClickListener(ClearButtonListener)

        val opListener = View.OnClickListener { v ->
            val op = (v as Button).text.toString()
            try {
                val value = newNumber.text.toString().toDouble()
                performOperation(value, op)
            } catch (e: NumberFormatException) {
                newNumber.setText("")
            }

            pendingOperation = op
            displayOperation.text = pendingOperation
        }

        Buttonequals.setOnClickListener(opListener)
        ButtonPlus.setOnClickListener(opListener)
        ButtonMinus.setOnClickListener(opListener)
        ButtonMultiply.setOnClickListener(opListener)
        ButtonDivide.setOnClickListener(opListener)
    }
    private fun performOperation(value: Double, operation: String) {
        if (operand1 == null) {
            operand1 = value
        } else {


            if (pendingOperation == "=") {
                pendingOperation = operation
            }

            when (pendingOperation) {
                "=" -> operand1 = value
                "+" -> operand1 = operand1!! + value
                "-" -> operand1 = operand1!! - value
                "*" -> operand1 = operand1!! * value
                "/" -> if (value == 0.0) {
                    operand1 = Double.NaN
                } else {
                    operand1 = operand1!! / value
                }
            }
        }
        result.setText(operand1.toString())
        newNumber.setText("")
    }
}

Answer 1

您从未重置operand1。因此，在获得第一个结果后，不会到达 performOperation 的第二行。然后，operand1 仍保留您在 when 子句中为 = 设置的最后一个值。在 result.setText(operand1.toString()) 之后，调用

if (pendingOperation == "=") {
    operand1 = null
}

这应该可以解决问题。