我開始與Android像2天前,請原諒我,如果我已經犯了一個天真的錯誤。我使用Java而不是XML的接口和應用程序崩潰每次啓動它時該emulator.Here是相同的堆棧跟蹤:應用程序崩潰在Android:運行時異常
03-01 21:08:46.941 7640-7640/com.example.prashant.wisdom E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.prashant.wisdom, PID: 7640
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.prashant.wisdom/com.example.prashant.wisdom.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.design.widget.FloatingActionButton.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.design.widget.FloatingActionButton.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.example.prashant.wisdom.MainActivity.onCreate(MainActivity.java:65)
at android.app.Activity.performCreate(Activity.java:6237)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
03-01 21:08:49.797 7640-7640/? I/Process: Sending signal. PID: 7640 SIG: 9
這裏是MainActivity.java:
package com.example.prashant.wisdom;
import android.graphics.Color;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.RelativeLayout;
import android.widget.Button;
import android.widget.EditText;
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
RelativeLayout layout=new RelativeLayout(this); //layout
Button button=new Button(this); //button
button.setText("SignIn");
//giving ids to layout and button
button.setId(1);
layout.setId(2);
//rules for button positioning
RelativeLayout.LayoutParams buttonDetails=new RelativeLayout.LayoutParams(
RelativeLayout.LayoutParams.WRAP_CONTENT,
RelativeLayout.LayoutParams.WRAP_CONTENT
); //the above snippet is to be written for every widget in the activity.
// For example,we'll write one for EditText also.
buttonDetails.addRule(RelativeLayout.CENTER_HORIZONTAL);
buttonDetails.addRule(RelativeLayout.CENTER_VERTICAL);
EditText username=new EditText(this); //username input
username.setId(3);
RelativeLayout.LayoutParams usernameDetails=new RelativeLayout.LayoutParams(
RelativeLayout.LayoutParams.WRAP_CONTENT,
RelativeLayout.LayoutParams.WRAP_CONTENT
); //rules for the EditText details.
//now we've to place the EditText above the button that is already positioned in the center.
//the following snippet does that.
usernameDetails.addRule(RelativeLayout.ABOVE,button.getId());
usernameDetails.addRule(RelativeLayout.CENTER_HORIZONTAL); //Rule for positioing in the center of the screen
usernameDetails.setMargins(0,0,0,50); //padding on all the sides.We're adding padding just to the bottom
//hence the rest are zero, but the bottom one is 50 pixels.
layout.addView(username); //EditText now positioned on the layout.
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
.setAction("Action", null).show();
}
});
setContentView(layout); //placement of layout itself
layout.addView(button, buttonDetails); //placement of buttons on the layout
}
@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);
}
}
這裏是content_main.xml:
<?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.example.prashant.wisdom.MainActivity"
tools:showIn="@layout/activity_main">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Hello World!" />
</RelativeLayout>
我懷疑是這個問題。如果fab沒有被定義,'R.id.fab'不應該被編譯。如果你看一下谷歌目前的模板,總是有一個帶有工具欄,側邊欄,動作按鈕等的佈局,其中包括內部佈局部分的內容佈局。 (我猜這就是這種情況) – F43nd1r