使用递归来解决用户输入的表达式（android 计算器应用程序）

Question

这是我的计算器应用程序的 MainActivity.java 代码：

package com.example.android.calculatorrecursion;
import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import java.util.ArrayList;

public class MainActivity extends ActionBarActivity {
    private String numString = "";
    private ArrayList<String> keyArrayList;
    EditText editTextView;
    Button button;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        editTextView = (EditText)findViewById(R.id.textView);
    }

    //when number buttons are clicked.
    public void onButtonClick(View v) {
        button = (Button) v;
        numString += button.getText().toString();
        editTextView.setText(numString);
    }

    //operation buttons which will also add a space before and after the      operator.
    public void onButtonSpaceGenerator(View view)
    {
        button = (Button)view;
        numString += " ";
        numString += button.getText().toString();
        numString += " ";
        editTextView.setText(numString);
    }

    public void onClear(View v){
        keyArrayList.clear();
        numString = "";
        editTextView.setText("");
    }

    public void createArray(){
        String tempString;
        int index;
        int begin = 0;
        while(begin < numString.length())
        {
          if(numString.contains(" ")) {
            index = numString.indexOf(" ", begin);
            tempString = numString.substring(begin, index);
            keyArrayList.add(tempString);
            begin += index + 1;
           }
        }
    }

    public void equals(View v){
        createArray();
        double total = calcResult(keyArrayList);
        editTextView.setText(Double.toString(total));
        numString = "";
    }

    public double calcResult(ArrayList<String> tempKeyArrayListArrayList){
         double tempNum Double.parseDouble(tempKeyArrayListArrayList.get(0));
         tempKeyArrayListArrayList.remove(0);
         if(tempKeyArrayListArrayList.size() >= 1) {
            String operator = tempKeyArrayListArrayList.get(0);
            tempKeyArrayListArrayList.remove(0);
            if (operator.equals("+"))
                return tempNum + calcResult(tempKeyArrayListArrayList);
            if (operator.equals("*"))
                return tempNum * calcResult(tempKeyArrayListArrayList);
            if (operator.equals("-"))
                return tempNum - calcResult(tempKeyArrayListArrayList);
            if (operator.equals("/"))
                return tempNum / calcResult(tempKeyArrayListArrayList);
         }
      return tempNum;
     }
}

但是，我目前有一个错误提示，

java.lang.IllegalStateException: Could not execute method of the activity
        at android.view.View.onClick(View.java:3679)
        at android.view.View.performClick(View.java:4203)
        at android.view.View$PerformClick.run(View.java:17189)
        at android.os.Handler.handleCallback(Handler.java:615)
        at android.os.Handler.dispatchMessage(Handler.java:92)
        at android.os.Looper.loop(Looper.java:137)
        at android.app.ActivityThread.main(ActivityThread.java:4950)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1004)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:771)
        at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.reflect.InvocationTargetException
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at android.view.View.onClick(View.java:3674)
        at android.view.View.performClick(View.java:4203)
        at android.view.View$PerformClick.run(View.java:17189)
        at android.os.Handler.handleCallback(Handler.java:615)
        at android.os.Handler.dispatchMessage(Handler.java:92)
        at android.os.Looper.loop(Looper.java:137)
        at android.app.ActivityThread.main(ActivityThread.java:4950)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1004)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:771)
        at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.NullPointerException
        at com.example.android.calculatorrecursion.MainActivity.createArray(MainActivity.java:57)
        at com.example.android.calculatorrecursion.MainActivity.equals(MainActivity.java:64)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at android.view.View.onClick(View.java:3674)
        at android.view.View.performClick(View.java:4203)
        at android.view.View$PerformClick.run(View.java:17189)
        at android.os.Handler.handleCallback(Handler.java:615)
        at android.os.Handler.dispatchMessage(Handler.java:92)
        at android.os.Looper.loop(Looper.java:137)
        at android.app.ActivityThread.main(ActivityThread.java:4950)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at   com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java
       :1004)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:771)
        at dalvik.system.NativeStart.main(Native Method)

而且我不确定为什么当我没有尝试初始化该行代码中的任何变量时计算机会抛出第三个错误。如果有人能够 explain/clarify 代码有什么问题以及前两个错误的含义，我将不胜感激。谢谢！

新错误：

    java.lang.IllegalStateException: Could not execute method of the activity
        at android.view.View.onClick(View.java:3679)
        at android.view.View.performClick(View.java:4203)
        at android.view.View$PerformClick.run(View.java:17189)
        at android.os.Handler.handleCallback(Handler.java:615)
        at android.os.Handler.dispatchMessage(Handler.java:92)
        at android.os.Looper.loop(Looper.java:137)
        at android.app.ActivityThread.main(ActivityThread.java:4950)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1004)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:771)
        at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.reflect.InvocationTargetException
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at android.view.View.onClick(View.java:3674)
        at android.view.View.performClick(View.java:4203)
        at android.view.View$PerformClick.run(View.java:17189)
        at android.os.Handler.handleCallback(Handler.java:615)
        at android.os.Handler.dispatchMessage(Handler.java:92)
        at android.os.Looper.loop(Looper.java:137)
        at android.app.ActivityThread.main(ActivityThread.java:4950)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1004)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:771)
        at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.IndexOutOfBoundsException: Invalid index 0, size is 0
        at java.util.ArrayList.throwIndexOutOfBoundsException(ArrayList.java:251)
        at java.util.ArrayList.get(ArrayList.java:304)
        at com.example.android.calculatorrecursion.MainActivity.calcResult(MainActivity.java:72)
        at com.example.android.calculatorrecursion.MainActivity.calcResult(MainActivity.java:82)
        at com.example.android.calculatorrecursion.MainActivity.equals(MainActivity.java:66)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at android.view.View.onClick(View.java:3674)
        at android.view.View.performClick(View.java:4203)
        at android.view.View$PerformClick.run(View.java:17189)
        at android.os.Handler.handleCallback(Handler.java:615)
        at android.os.Handler.dispatchMessage(Handler.java:92)
        at android.os.Looper.loop(Looper.java:137)
        at android.app.ActivityThread.main(ActivityThread.java:4950)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1004)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:771)
        at dalvik.system.NativeStart.main(Native Method)

Answer 1

您收到 NullPointerException，因为您从未初始化 keyArrayList。

NPE 发生在这一行：keyArrayList.add(tempString);

只需在onCreate()中初始化即可。

public class MainActivity extends ActionBarActivity {
    private String numString = "";
    private ArrayList<String> keyArrayList;
    EditText editTextView;
    Button button;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        editTextView = (EditText)findViewById(R.id.textView);

        keyArrayList = new ArrayList<String>(); //added
    }

建议阅读：What is a NullPointerException, and how do I fix it?

使用递归来解决用户输入的表达式（android 计算器应用程序）

Using recursion to solve user entered expressions (android calculator app)

java

android

calculator