Question

我是Android开发的新手，并尝试制作一个简单的计算器应用程序。我改变的最后一件事是尝试将两个字符串连接成一个并在EditText中显示新字符串。当我在Android Studio中查看它时，我的项目中没有错误，但是当我在手机上运行应用程序时它停止工作。这是电话问题还是......？

   package com.example.jeff.calculator;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
    double total = 0;
    double transaction = 0;
    String Back = "";
    String currentString = "";
    String transactionString = "";
    EditText calcInput = (EditText) findViewById(R.id.EtCalc);

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

        buttonSetup();
    }

    private void concatenation() {
        transactionString = transactionString + currentString;
        calcInput.setText(String.valueOf(transactionString));
    }

    private void buttonSetup() {

        Button button0 = (Button) findViewById(R.id.Btn0);
        Button button1 = (Button) findViewById(R.id.Btn1);
        Button button2 = (Button) findViewById(R.id.Btn2);
        Button button3 = (Button) findViewById(R.id.Btn3);
        Button button4 = (Button) findViewById(R.id.Btn4);
        Button button5 = (Button) findViewById(R.id.Btn5);
        Button button6 = (Button) findViewById(R.id.Btn6);
        Button button7 = (Button) findViewById(R.id.Btn7);
        Button button8 = (Button) findViewById(R.id.Btn8);
        Button button9 = (Button) findViewById(R.id.Btn9);

        button0.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                currentString = "0";
                calcInput.setText(String.valueOf(transactionString));
            }
        });
        button1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                currentString = "1";
                concatenation();
            }
        });
        button2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                currentString = "2";
                concatenation();
            }
        });
        button3.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                currentString = "3";
                concatenation();
            }
        });
        button4.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                currentString = "4";
                concatenation();
            }
        });
        button5.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                currentString = "5";
                concatenation();
            }
        });
        button6.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                currentString = "6";
                concatenation();
            }
        });
        button7.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                currentString = "7";
                concatenation();
            }
        });
        button8.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                currentString = "8";
                concatenation();
            }
        });
        button9.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                currentString = "9";
                concatenation();
            }
        });
        // (EditText) inputTxt = (EditText) findViewById(R.id.input);

// Store EditText in Variable

        // String str = inputTxt.getText().toString();
    }

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

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

logcat的

   09-07 23:54:12.649  22823-22823/com.example.jeff.calculator I/art﹕ Late-enabling -Xcheck:jni
09-07 23:54:12.718  22823-22823/com.example.jeff.calculator D/AndroidRuntime﹕ Shutting down VM
09-07 23:54:12.719  22823-22823/com.example.jeff.calculator E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.jeff.calculator, PID: 22823
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.jeff.calculator/com.example.jeff.calculator.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2290)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2442)
            at android.app.ActivityThread.access$800(ActivityThread.java:156)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1351)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:211)
            at android.app.ActivityThread.main(ActivityThread.java:5373)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1020)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:815)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
            at android.app.Activity.findViewById(Activity.java:2072)
            at com.example.jeff.calculator.MainActivity.<init>(MainActivity.java:22)
            at java.lang.reflect.Constructor.newInstance(Native Method)
            at java.lang.Class.newInstance(Class.java:1606)
            at android.app.Instrumentation.newActivity(Instrumentation.java:1066)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2280)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2442)
            at android.app.ActivityThread.access$800(ActivityThread.java:156)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1351)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:211)
            at android.app.ActivityThread.main(ActivityThread.java:5373)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1020)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:815)

Answer 1

除非特别说明，否则不要在调用Activity之后调用从super.onCreate()继承的方法。

此外，在致电findViewById()之前，您无法致电setContentView()，因为无法找到小部件。

替换：

EditText calcInput = (EditText) findViewById(R.id.EtCalc);

使用：

EditText calcInput;

并添加：

calcInput = (EditText) findViewById(R.id.EtCalc);

在setContentView()致电onCreate()后

。

Answer 2

您正在定义变量

EditText calcInput = (EditText) findViewById(R.id.EtCalc);

这意味着当初始化类时，您正试图在null窗口中找到一个元素。

你不能这样做。

只需使用：

EditText calcInput;

然后在onCreate方法中初始化变量。

类似的东西：

public class MainActivity extends AppCompatActivity {
    //...
    EditText calcInput;

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

        buttonSetup();

        calcInput = (EditText) findViewById(R.id.EtCalc);
    }

}

Answer 3

你必须移动这一行：

EditText calcInput = (EditText) findViewById(R.id.EtCalc);

以下行：

setContentView(R.layout.activity_main);

Android Studio中没有错误，但应用程序崩溃

3 个答案: