简单的程序部队关闭

Question

我正在使用Eclipse为Android制作程序。这里是我的MainAcitvity.java代码：

package com.example.changeme; 
import android.app.Activity; 
import android.os.Bundle; 
import android.view.Menu; 
import android.view.View; 
import android.view.View.OnClickListener; 
import android.widget.Button; 
import android.widget.EditText; 
import android.widget.TextView; 

public class MainActivity extends Activity { 

    Button set = (Button)findViewById(R.id.set); 
    TextView out = (TextView)findViewById(R.id.out); 
    EditText in = (EditText)findViewById(R.id.input); 

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

     set.setOnClickListener(new OnClickListener() { 
      public void onClick(View v) { 
       setcontent(); 
      } 
     }); 
    } 
    public void setcontent() 
    { 
     String con = in.getText().toString(); 
     out.setText(con); 
    } 

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

} 

布局主要活动：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
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" 
tools:context=".MainActivity" > 

<TextView 
    android:id="@+id/out" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_alignLeft="@+id/set" 
    android:layout_alignParentBottom="true" 
    android:layout_marginBottom="37dp" 
    android:text="@string/Change" 
    android:textAppearance="?android:attr/textAppearanceLarge" /> 

<Button 
    android:id="@+id/set" 
    style="?android:attr/buttonStyleSmall" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_below="@+id/input" 
    android:layout_centerHorizontal="true" 
    android:layout_marginTop="18dp" 
    android:text="@string/Set" /> 

<EditText 
    android:id="@+id/input" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_alignParentTop="true" 
    android:layout_centerHorizontal="true" 
    android:layout_marginTop="37dp" 
    android:ems="10" 
    android:inputType="textPersonName" /> 

，当我跑我的N4程序经常死机。

Answer 1

您需要充气布局之前检索您的内容，否则findViewById将返回null，因此线set.setOnClickListener(/**/);将抛出一个NullPointerException，使您的应用程序崩溃。

Button set; 
TextView out; 
EditText in; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); //<-- the layout is inflated, i.e it's rendered 

    /******************************************************/ 
    //Now you can retrieve your elements from this layout 
    set = (Button)findViewById(R.id.set); 
    out = (TextView)findViewById(R.id.out); 
    in = (EditText)findViewById(R.id.input); 
    /*****************************************************/ 

    set.setOnClickListener(new OnClickListener() { 
     public void onClick(View v) { 
      setcontent(); 
     } 
    }); 
}