Question

我是Android Studio的新手，我知道一点java.So我正在尝试一个简单布局的简单计算器应用程序。我希望我的所有代码都是正确的，但我不知道它有什么问题。当时我运行应用程序它根本没有打开并显示一个FATAL异常main.Here是我的app的xml代码，java代码和logcat。所以请帮我修正。谢谢！

XML Code ::

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayoutxmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@color/colorPrimary"
tools:context="com.example.narendra.calculator.MainActivity"
tools:layout_editor_absoluteY="81dp"
tools:layout_editor_absoluteX="0dp">


<EditText
    android:id="@+id/editText"
    android:layout_width="360dp"
    android:layout_height="159dp"
    android:background="@android:color/holo_blue_light"
    android:ems="10"
    android:hint="@string/num"
    android:inputType="number"
    android:textAlignment="center"
    android:textAllCaps="true"
    android:textSize="50sp"
    android:textStyle="bold"
    tools:ignore="MissingConstraints,RtlHardcoded"
    app:layout_constraintTop_toTopOf="parent"
    android:layout_marginTop="16dp"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintLeft_toLeftOf="parent"
    android:layout_marginRight="8dp"
    android:layout_marginLeft="8dp"
    android:layout_marginStart="8dp"
    android:layout_marginEnd="8dp" />

<EditText
    android:id="@+id/editText2"
    android:layout_width="360dp"
    android:layout_height="159dp"
    android:background="@android:color/holo_green_light"
    android:ems="10"
    android:hint="@string/Num2"
    android:inputType="number"
    android:textAlignment="center"
    android:textAllCaps="true"
    android:textSize="50sp"
    android:textStyle="bold"
    tools:ignore="MissingConstraints,RtlHardcoded"
    tools:layout_editor_absoluteY="180dp"
    tools:layout_editor_absoluteX="26dp" />

<Button
    android:id="@+id/button"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:onClick="onAddButtonClick"
    android:text="@string/a"
    android:textSize="50sp"
    app:layout_constraintLeft_toLeftOf="@+id/editText2"
    app:layout_constraintTop_toTopOf="@+id/editText"
    android:layout_marginTop="323dp"
    tools:ignore="RtlHardcoded"
    app:layout_constraintRight_toLeftOf="@+id/button3"
    app:layout_constraintHorizontal_bias="0.0"
    android:layout_marginRight="5dp"
    android:layout_marginEnd="5dp" />

<Button
    android:id="@+id/button2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:onClick="onSubtractionButtonClick"
    android:text="@string/m"
    android:textSize="50sp"
    android:layout_marginLeft="5dp"
    app:layout_constraintTop_toTopOf="@+id/editText"
    android:layout_marginTop="323dp"
    tools:ignore="RtlHardcoded"
    app:layout_constraintLeft_toRightOf="@+id/button"
    app:layout_constraintHorizontal_bias="0.03"
    android:layout_marginRight="5dp"
    app:layout_constraintRight_toRightOf="@+id/button3"
    android:layout_marginStart="5dp"
    android:layout_marginEnd="5dp" />

<Button
    android:id="@+id/button3"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:onClick="onMultiplicationButtonClick"
    android:text="@string/p"
    android:textSize="50sp"
    android:layout_marginLeft="5dp"
    app:layout_constraintTop_toTopOf="@+id/editText"
    android:layout_marginTop="323dp"
    tools:ignore="RtlHardcoded"
    app:layout_constraintLeft_toRightOf="@+id/button2"
    android:layout_marginRight="5dp"
    app:layout_constraintRight_toRightOf="@+id/button4"
    app:layout_constraintHorizontal_bias="0.0"
    android:layout_marginStart="5dp"
    android:layout_marginEnd="5dp" />

<Button
    android:id="@+id/button4"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:onClick="onDivisionButtonClick"
    android:text="@string/d"
    android:textSize="50sp"
    android:layout_marginLeft="5dp"
    app:layout_constraintRight_toRightOf="@+id/editText2"
    app:layout_constraintTop_toTopOf="@+id/editText"
    android:layout_marginTop="323dp"
    tools:ignore="RtlHardcoded"
    app:layout_constraintLeft_toRightOf="@+id/button3"
    app:layout_constraintHorizontal_bias="0.0"
    android:layout_marginStart="5dp"
    android:layout_marginRight="5dp" />

<TextView
    android:id="@+id/textView"
    android:layout_width="360dp"
    android:layout_height="100dp"
    android:layout_marginBottom="8dp"
    android:layout_marginLeft="8dp"
    android:layout_marginRight="16dp"
    android:layout_marginTop="8dp"
    android:background="@color/colorAccent"
    android:text="@string/Result"
    android:textAlignment="center"
    android:textAllCaps="true"
    android:textSize="50sp"
    android:textStyle="bold"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintHorizontal_bias="0.771"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    app:layout_constraintVertical_bias="0.839"
    tools:ignore="RtlHardcoded" />

Java Code ::

package com.example.narendra.calculator;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
}
EditText e1=(EditText)findViewById(R.id.editText);
EditText e2=(EditText)findViewById(R.id.editText2);
TextView t1=(TextView)findViewById(R.id.textView);
int num1=Integer.parseInt(e1.getText().toString());
int num2=Integer.parseInt(e2.getText().toString());
public void conditionCheck(){
    if(e1.getText().toString().isEmpty()){
        e1.setError("Please Enter a valid number");
    }
    else if(e2.getText().toString().isEmpty()){
        e2.setError("Please Enter a valid number");
    }
}
public void onAddButtonClick(View v){
    conditionCheck();
    int sum=num1+num2;
    t1.setText(sum);
    Toast.makeText(MainActivity.this, " Thank You ", Toast.LENGTH_SHORT).show();
}
public void onSubtractionButtonClick(View v){
    conditionCheck();
    int diff=num1-num2;
    t1.setText(diff);
    Toast.makeText(MainActivity.this, " Thank You ", Toast.LENGTH_SHORT).show();
}
public void onMultiplicationButtonClick(View v){
    conditionCheck();
    int product=num1*num2;
    t1.setText(product);
    Toast.makeText(MainActivity.this, " Thank You ", Toast.LENGTH_SHORT).show();
}
public void onDivisionButtonClick(View v){
    conditionCheck();
    if(num2==0){
        e2.setError(" Zero is not allowed here");
    }
    else {
        int quotient = num1 / num2;
        t1.setText(quotient);
        Toast.makeText(MainActivity.this, " Thank You ", Toast.LENGTH_SHORT).show();
    }
}

}

Logcat ::

    06-14 12:31:23.216 4822-4822/? I/art: Not late-enabling -Xcheck:jni (already on)
    06-14 12:31:23.216 4822-4822/? W/art: Unexpected CPU variant for X86 using defaults: x86
    06-14 12:31:23.342 4822-4822/? W/System: ClassLoader referenced unknown path: /data/app/com.example.narendra.calculator-2/lib/x86
    06-14 12:31:23.348 4822-4822/? I/InstantRun: starting instant run server: is main process
    06-14 12:31:23.405 4822-4822/? D/AndroidRuntime: Shutting down VM
    06-14 12:31:23.405 4822-4822/? E/AndroidRuntime: FATAL EXCEPTION: main
                                             Process: com.example.narendra.calculator, PID: 4822
                                             java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.narendra.calculator/com.example.narendra.calculator.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
                                                 at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2548)
                                                 at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
                                                 at android.app.ActivityThread.-wrap12(ActivityThread.java)
                                                 at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
                                                 at android.os.Handler.dispatchMessage(Handler.java:102)
                                                 at android.os.Looper.loop(Looper.java:154)
                                                 at android.app.ActivityThread.main(ActivityThread.java:6077)
                                                 at java.lang.reflect.Method.invoke(Native Method)
                                                 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866)
                                                 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)
                                              Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
                                                 at android.support.v7.app.AppCompatDelegateImplBase.<init>(AppCompatDelegateImplBase.java:120)
                                                 at android.support.v7.app.AppCompatDelegateImplV9.<init>(AppCompatDelegateImplV9.java:155)
                                                 at android.support.v7.app.AppCompatDelegateImplV11.<init>(AppCompatDelegateImplV11.java:31)
                                                 at android.support.v7.app.AppCompatDelegateImplV14.<init>(AppCompatDelegateImplV14.java:55)
                                                 at android.support.v7.app.AppCompatDelegateImplV23.<init>(AppCompatDelegateImplV23.java:33)
                                                 at android.support.v7.app.AppCompatDelegateImplN.<init>(AppCompatDelegateImplN.java:33)
                                                 at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:201)
                                                 at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:185)
                                                 at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:519)
                                                 at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:190)
                                                 at com.example.narendra.calculator.MainActivity.<init>(MainActivity.java:17)
                                                 at java.lang.Class.newInstance(Native Method)
                                                 at android.app.Instrumentation.newActivity(Instrumentation.java:1078)
                                                 at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2538)
                                                 at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) 
                                                 at android.app.ActivityThread.-wrap12(ActivityThread.java) 
                                                 at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) 
                                                 at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                 at android.os.Looper.loop(Looper.java:154) 
                                                 at android.app.ActivityThread.main(ActivityThread.java:6077) 
                                                 at java.lang.reflect.Method.invoke(Native Method) 
                                                 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866) 
                                                 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)

Answer 1

你的onCreate（）提早关闭

将其更改为

package com.example.narendra.calculator;

    import android.support.v7.app.AppCompatActivity;
    import android.os.Bundle;
    import android.view.View;
    import android.widget.EditText;
    import android.widget.TextView;
    import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
EditText e1,e2;
int num1,num2;
TextView t1;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

     e1 = (EditText) findViewById(R.id.editText);
     e2 = (EditText) findViewById(R.id.editText2);
     t1 = (TextView) findViewById(R.id.textView);
     num1 = Integer.parseInt(e1.getText().toString());
     num2 = Integer.parseInt(e2.getText().toString());
} public void conditionCheck(){
    if(e1.getText().toString().isEmpty()){
        e1.setError("Please Enter a valid number");
    }
    else if(e2.getText().toString().isEmpty()){
        e2.setError("Please Enter a valid number");
    }
}
public void onAddButtonClick(View v){
    conditionCheck();
    int sum=num1+num2;
    t1.setText(sum);
    Toast.makeText(MainActivity.this, " Thank You ", Toast.LENGTH_SHORT).show();
}
public void onSubtractionButtonClick(View v){
    conditionCheck();
    int diff=num1-num2;
    t1.setText(diff);
    Toast.makeText(MainActivity.this, " Thank You ", Toast.LENGTH_SHORT).show();
}
public void onMultiplicationButtonClick(View v){
    conditionCheck();
    int product=num1*num2;
    t1.setText(product);
    Toast.makeText(MainActivity.this, " Thank You ", Toast.LENGTH_SHORT).show();
}
public void onDivisionButtonClick(View v){
    conditionCheck();
    if(num2==0){
        e2.setError(" Zero is not allowed here");
    }
    else {
        int quotient = num1 / num2;
        t1.setText(quotient);
        Toast.makeText(MainActivity.this, " Thank You ", Toast.LENGTH_SHORT).show();
    }
}

}

Answer 2

只需修改（view init）位置的代码，像这样：

MainActivity{
   onCreate(){
    View view =    findViewById(id);
  }
}

注意：Activity是具有自我生命周期的组件。所以不能将其应用为简单的Pojo

FATAL EXCEPTION：main .... java.lang.RuntimeException：无法实例化活动ComponentInfo。

2 个答案: