Question

此应用只需点击Button即可点击TextBox更改其内容。

MainActivity.java

package com.lost.eventhandling;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    Button willybutton=(Button)findViewById(R.id.willybutton);
    assert willybutton != null;
    willybutton.setOnClickListener(
            new Button.OnClickListener(){
                public void onClick(View v){
                    TextView willytext=(TextView)findViewById(R.id.willytext);
                    willytext.setText("I DID IT ");
                }
            }
    );
}

@Override
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);
   }
}

现在我有xml代码

 <?xml version="1.0" encoding="utf-8"?>
 <RelativeLayout xmlns: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:paddingBottom="@dimen/activity_vertical_margin"
 android:paddingLeft="@dimen/activity_horizontal_margin"
 android:paddingRight="@dimen/activity_horizontal_margin"
 android:paddingTop="@dimen/activity_vertical_margin"
 app:layout_behavior="@string/appbar_scrolling_view_behavior"
 tools:context="com.lost.eventhandling.MainActivity"
 android:background="#71f8bd">

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:textAppearance="?android:attr/textAppearanceLarge"
    android:text="@string/willy_text"
    android:id="@+id/willytext"
    android:textColor="#e83858"
    android:layout_alignParentTop="true"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="179dp" />

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/willy_button"
    android:id="@+id/willybutton"
    android:layout_centerVertical="true"
    android:layout_centerHorizontal="true"
    android:background="#764045"/>

    </RelativeLayout>

每当我调试我的应用程序时，它都会发送一条不必要的消息。我的安卓监视器给了我这个红色的文字：

FATAL EXCEPTION: main
 Process: com.lost.eventhandling, PID: 2449
 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.lost.eventhandling/com.lost.eventhandling.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
     at android.app.ActivityThread.access$800(ActivityThread.java:151)
     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
     at android.os.Handler.dispatchMessage(Handler.java:102)
     at android.os.Looper.loop(Looper.java:135)
     at android.app.ActivityThread.main(ActivityThread.java:5254)
     at java.lang.reflect.Method.invoke(Native Method)
     at java.lang.reflect.Method.invoke(Method.java:372)
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
  Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
     at com.lost.eventhandling.MainActivity.onCreate(MainActivity.java:26)
     at android.app.Activity.performCreate(Activity.java:5990)
     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387) 
     at android.app.ActivityThread.access$800(ActivityThread.java:151) 
     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303) 
     at android.os.Handler.dispatchMessage(Handler.java:102) 
     at android.os.Looper.loop(Looper.java:135) 
     at android.app.ActivityThread.main(ActivityThread.java:5254) 
     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:903) 
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

Answer 1

使用以下代码行替换OnclickListener Button代码：

 Button willybutton=(Button)findViewById(R.id.willybutton);
 TextView willytext=(TextView)findViewById(R.id.willytext);

            willybutton.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
            /////PERFORM YOURS OPERATION HERE


                }
            });

我刚刚开始Android开发，当我制作这个应用程序时，它在调试时没有工作

1 个答案: