我是 Android 开发的新手，我正在尝试制作一个单位转换应用程序。我的距离 Activity 能够自行展开，但当我尝试添加 ClickListener 时，一切都会变得糟糕。我收到一个 NullPointerException，我不知道为什么，我的第一个想法是我引用了错误的 Id，但看起来我引用了正确的位置。该应用程序启动到主屏幕，其中有一个用于打开“距离” Activity 的按钮。当我尝试打开距离时，应用程序崩溃了。这是距离:
package org.belliveau.convert;
import android.annotation.TargetApi;
import android.app.Activity;
import android.os.Build;
import android.os.Bundle;
import android.support.v4.app.NavUtils;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
public class Distance extends Activity {
int inputInInches, output;
Button inchInput, footInput, yardInput, mileInput, inchOutput, footOutput, yardOutput, mileOutput;
TextView result;
EditText inputText;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setButtons();
setContentView(R.layout.activity_distance);
setClickListeners();
// Show the Up button in the action bar.
setupActionBar();
}
/**
* Set up the {@link android.app.ActionBar}, if the API is available.
*/
@TargetApi(Build.VERSION_CODES.HONEYCOMB)
private void setupActionBar() {
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
getActionBar().setDisplayHomeAsUpEnabled(true);
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.distance, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
switch (item.getItemId()) {
case android.R.id.home:
// This ID represents the Home or Up button. In the case of this
// activity, the Up button is shown. Use NavUtils to allow users
// to navigate up one level in the application structure. For
// more details, see the Navigation pattern on Android Design:
//
// http://developer.android.com/design/patterns/navigation.html#up-vs-back
//
NavUtils.navigateUpFromSameTask(this);
return true;
}
return super.onOptionsItemSelected(item);
}
private void setButtons(){
inchInput = (Button) findViewById(R.id.inch);
footInput = (Button) findViewById(R.id.foot_button);
yardInput = (Button) findViewById(R.id.yard_button);
mileInput = (Button) findViewById(R.id.mile_button);
inchOutput = (Button) findViewById(R.id.inch_post);
footOutput = (Button) findViewById(R.id.foot_post);
yardOutput = (Button) findViewById(R.id.yard_post);
mileOutput = (Button) findViewById(R.id.mile_post);
result = (TextView) findViewById(R.id.dist_result);
inputText = (EditText) findViewById(R.id.distance_initial);
}
private void setClickListeners(){
inchInput.setOnClickListener(new OnClickListener(){
public void onClick(View v) {
int input = Integer.valueOf(inputText.getText().toString());
inch2inch(input);
}
});
footInput.setOnClickListener(new OnClickListener(){
public void onClick(View v) {
int input = Integer.valueOf(inputText.getText().toString());
foot2inch(input);
}
});
yardInput.setOnClickListener(new OnClickListener(){
public void onClick(View v) {
int input = Integer.valueOf(inputText.getText().toString());
yard2inch(input);
}
});
mileInput.setOnClickListener(new OnClickListener(){
public void onClick(View v) {
int input = Integer.valueOf(inputText.getText().toString());
mile2inch(input);
}
});
inchOutput.setOnClickListener(new OnClickListener(){
public void onClick(View v) {
inchDuh();
}
});
footOutput.setOnClickListener(new OnClickListener(){
public void onClick(View v) {
outputFoot();
}
});
yardOutput.setOnClickListener(new OnClickListener(){
public void onClick(View v) {
outputYard();
}
});
mileOutput.setOnClickListener(new OnClickListener(){
public void onClick(View v) {
outputMile();
}
});
}
public void inch2inch(int inch) {
inputInInches = inch;
}
public void foot2inch(int foot) {
inputInInches = foot * 12;
}
public void yard2inch(int yard) {
int foot = yard * 3;
inputInInches = foot * 3;
}
public void mile2inch(int mile) {
int yard = mile * 1760;
int foot = yard * 3;
inputInInches = foot * 3;
}
public void outputFoot() {
output = inputInInches / 12;
result.setText(output);
}
public void outputYard() {
int foot = inputInInches / 12;
output = foot / 3;
result.setText(output);
}
public void outputMile() {
int foot = inputInInches / 12;
int yard = foot / 3;
output = yard / 1760;
result.setText(output);
}
public void inchDuh(){
output = inputInInches;
result.setText(output);
}
和 LogCat:
12-20 21:34:35.674: I/Adreno-EGL(3199): <qeglDrvAPI_eglInitialize:316>: EGL 1.4 QUALCOMM build: (CL4169980)
12-20 21:34:35.674: I/Adreno-EGL(3199): OpenGL ES Shader Compiler Version: 17.01.10.SPL
12-20 21:34:35.674: I/Adreno-EGL(3199): Build Date: 10/30/13 Wed
12-20 21:34:35.674: I/Adreno-EGL(3199): Local Branch:
12-20 21:34:35.674: I/Adreno-EGL(3199): Remote Branch:
12-20 21:34:35.674: I/Adreno-EGL(3199): Local Patches:
12-20 21:34:35.674: I/Adreno-EGL(3199): Reconstruct Branch:
12-20 21:34:35.744: D/OpenGLRenderer(3199): Enabling debug mode 0
12-20 21:34:38.474: D/AndroidRuntime(3199): Shutting down VM
12-20 21:34:38.474: W/dalvikvm(3199): threadid=1: thread exiting with uncaught exception (group=0x4166dd28)
12-20 21:34:38.476: E/AndroidRuntime(3199): FATAL EXCEPTION: main
12-20 21:34:38.476: E/AndroidRuntime(3199): Process: org.belliveau.convert, PID: 3199
12-20 21:34:38.476: E/AndroidRuntime(3199): java.lang.RuntimeException: Unable to start activity ComponentInfo{org.belliveau.convert/org.belliveau.convert.Distance}: java.lang.NullPointerException
12-20 21:34:38.476: E/AndroidRuntime(3199): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2190)
12-20 21:34:38.476: E/AndroidRuntime(3199): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2250)
12-20 21:34:38.476: E/AndroidRuntime(3199): at android.app.ActivityThread.access$700(ActivityThread.java:139)
12-20 21:34:38.476: E/AndroidRuntime(3199): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1411)
12-20 21:34:38.476: E/AndroidRuntime(3199): at android.os.Handler.dispatchMessage(Handler.java:102)
12-20 21:34:38.476: E/AndroidRuntime(3199): at android.os.Looper.loop(Looper.java:137)
12-20 21:34:38.476: E/AndroidRuntime(3199): at android.app.ActivityThread.main(ActivityThread.java:5083)
12-20 21:34:38.476: E/AndroidRuntime(3199): at java.lang.reflect.Method.invokeNative(Native Method)
12-20 21:34:38.476: E/AndroidRuntime(3199): at java.lang.reflect.Method.invoke(Method.java:515)
12-20 21:34:38.476: E/AndroidRuntime(3199): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777)
12-20 21:34:38.476: E/AndroidRuntime(3199): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:593)
12-20 21:34:38.476: E/AndroidRuntime(3199): at de.robv.android.xposed.XposedBridge.main(XposedBridge.java:126)
12-20 21:34:38.476: E/AndroidRuntime(3199): at dalvik.system.NativeStart.main(Native Method)
12-20 21:34:38.476: E/AndroidRuntime(3199): Caused by: java.lang.NullPointerException
12-20 21:34:38.476: E/AndroidRuntime(3199): at org.belliveau.convert.Distance.setClickListeners(Distance.java:82)
12-20 21:34:38.476: E/AndroidRuntime(3199): at org.belliveau.convert.Distance.onCreate(Distance.java:28)
12-20 21:34:38.476: E/AndroidRuntime(3199): at android.app.Activity.performCreate(Activity.java:5260)
12-20 21:34:38.476: E/AndroidRuntime(3199): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1110)
12-20 21:34:38.476: E/AndroidRuntime(3199): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2154)
12-20 21:34:38.476: E/AndroidRuntime(3199): ... 12 more
最佳答案
反转这些调用
setButtons();
setContentView(R.layout.activity_distance);
换句话说，它们应该是
setContentView(R.layout.activity_distance);
setButtons();
您正在调用 setButtons() ，它会尝试从统一的 UI 获取 View ，因此
inchInput = (Button) findViewById(R.id.inch); // returns null
将 inchInput 设置为 null。
关于java - 尝试在 Android 中添加 ClickListener 时出现 NullPointerException.. 为什么？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/20714840/