java - Android EditText 到 int 对话错误

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 7 年前。

这可能是问得最多的问题，但很难找到答案。首先我是新手。我想制作简单的二次方程公式应用程序。这将使我们能够快速找到解决方案。我遇到了 Android Studio 说代码没问题，但打开应用程序后设备崩溃的问题。

private Button   mButton;
private EditText mEdit;
private EditText mEdit1;
private EditText mEdit2;
private TextView mText;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    mButton = (Button)findViewById(R.id.button2);
    mEdit   = (EditText)findViewById(R.id.editText);
    mEdit1   = (EditText)findViewById(R.id.editText2);
    mEdit2   = (EditText)findViewById(R.id.editText3);
    mText = (TextView)findViewById(R.id.textView4);

    //Int definēšana
    final int i1 = Integer.parseInt(mEdit.getText().toString());
    final int i2 = Integer.parseInt(mEdit1.getText().toString());
    final int i3 = Integer.parseInt(mEdit2.getText().toString());

    mButton.setOnClickListener(
            new View.OnClickListener()
            {
                public void onClick(View view)
                {
                    //Mainīgo ievade
                    int a = i1;
                    //Pārbauda vai a nav nulle
                    if (a == 0){
                        mText.setText(String.valueOf("Nav kvadrātvienādojums"));
                    } else {
                        int b = i2;
                        int c = i3;

                        //Diskriminanta aprēķināšana
                        double diskr = (b*b)-4*a*c;
                        //Kvadrātsakne no diskriminanta
                        double sd = (double) Math.sqrt(diskr);
                        //Sakņu aprēķināšana
                        double x1 = (-b+sd)/(2*a);
                        double x2 = (-b-sd)/(2*a);

                        //Rezultāta izvade
                        if (diskr < 0){
                            mText.setText(String.valueOf("Kvadrātvienādojumam nav sakņu"));
                        } else if (diskr == 0){
                            mText.setText(String.valueOf("Kvadrātvienādojumam ir viena sakne: " + x1));
                        } else {
                            mText.setText(String.valueOf("Kvadrātvienādojuma saknes: " + x1 + " un " + x2));
                        }
                    }
                }
            });
} 

和 logcat 可以帮助检测问题

FATAL EXCEPTION: main
Process: com.homemade.prtbust.kvadratvienadojums, PID: 4552
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.homem`enter code here`ade.prtbust.kvadratvienadojums/com.homemade.prtbust.kvadratvienadojums.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.text.Editable android.widget.EditText.getText()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3133)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3243)
at android.app.ActivityThread.access$1000(ActivityThread.java:218)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1718)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(ActivityThread.java:6917)
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:1404)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.text.Editable android.widget.EditText.getText()' on a null object reference
    at com.homemade.prtbust.kvadratvienadojums.MainActivity.onCreate(MainActivity.java:38)
    at android.app.Activity.performCreate(Activity.java:6609)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1134)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3086)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3243)
    at android.app.ActivityThread.access$1000(ActivityThread.java:218)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1718)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:145)
    at android.app.ActivityThread.main(ActivityThread.java:6917)
    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:1404
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199) 

Answer 1

我想你会错过

setContentView(R.layout.activity_main); "Activity_name 是编辑文本所在的布局名称"

setContentView 必须在使用 findViewById

之前调用