使用Java代码创建相对布局时出错

概述我是Android开发新手,并尝试使用Java代码创建RelativeLayout.下面是我的MainActivty.javapackagecom.rt.droid.rellayoutjava;importandroid.app.Activity;importandroid.graphics.Color;importandroid.os.Bundle;importandroid.widget.Button;importandroid.widget

我是Android开发新手,并尝试使用Java代码创建relativeLayout.
下面是我的MainActivty.java

package com.rt.droID.rellayoutjava;import androID.app.Activity;import androID.graphics.color;import androID.os.Bundle;import androID.Widget.button;import androID.Widget.EditText;import androID.Widget.relativeLayout;import androID.Widget.TextVIEw;import androID.Widget.relativeLayout.LayoutParams;public class MainActivity extends Activity {relativeLayout r;public static LayoutParams msgDim;EditText usernameVal, passwordVal;TextVIEw message, username, password;button login;@OverrIDeprotected voID onCreate(Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    init();    createMessage();    r.addVIEw(message,msgDim);    setContentVIEw(r);}public voID init() {    r = new relativeLayout(this);    LayoutParams dimensions = new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT);    r.setLayoutParams(dimensions);    usernameVal = new EditText(this);    passwordVal = new EditText(this);    message=new TextVIEw(this);    username = new TextVIEw(this);    password = new TextVIEw(this);    login = new button(this);}public voID createMessage(){    LayoutParams msgDim = new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);    msgDim.addRule(relativeLayout.AliGN_PARENT_left);    message.setLayoutParams(msgDim);    message.setText("Please login first");    message.setTextcolor(color.rgb(0,0,0));}}

但是我得到空指针异常(从Logcat下面)

 Process: com.rt.droID.rellayoutjava, PID: 2471java.lang.RuntimeException: Unable to start activity ComponentInfo{com.rt.droID.rellayoutjava/com.rt.droID.rellayoutjava.MainActivity}: java.lang.NullPointerException: Attempt to read from fIEld 'int androID.vIEw.VIEwGroup$LayoutParams.wIDth' on a null object reference        at androID.app.ActivityThread.performlaunchActivity(ActivityThread.java:2298)        at androID.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)        at androID.app.ActivityThread.access0(ActivityThread.java:144)        at androID.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)        at androID.os.Handler.dispatchMessage(Handler.java:102)        at androID.os.Looper.loop(Looper.java:135)        at androID.app.ActivityThread.main(ActivityThread.java:5221)        at java.lang.reflect.Method.invoke(Native Method)        at java.lang.reflect.Method.invoke(Method.java:372)        at com.androID.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)        at com.androID.internal.os.ZygoteInit.main(ZygoteInit.java:694) Caused by: java.lang.NullPointerException: Attempt to read from fIEld 'int androID.vIEw.VIEwGroup$LayoutParams.wIDth' on a null object reference        at androID.vIEw.VIEwGroup$LayoutParams.<init>(VIEwGroup.java:6403)        at androID.vIEw.VIEwGroup$marginLayoutParams.<init>(VIEwGroup.java:6685)        at androID.Widget.relativeLayout$LayoutParams.<init>(relativeLayout.java:1345)        at androID.Widget.relativeLayout.generateLayoutParams(relativeLayout.java:1104)        at androID.vIEw.VIEwGroup.addVIEwInner(VIEwGroup.java:3889)        at androID.vIEw.VIEwGroup.addVIEw(VIEwGroup.java:3733)        at androID.vIEw.VIEwGroup.addVIEw(VIEwGroup.java:3709)        at com.rt.droID.rellayoutjava.MainActivity.onCreate(MainActivity.java:24)        at androID.app.Activity.performCreate(Activity.java:5933)        at androID.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)        at androID.app.ActivityThread.performlaunchActivity(ActivityThread.java:2251)            at androID.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)            at androID.app.ActivityThread.access0(ActivityThread.java:144)            at androID.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)            at androID.os.Handler.dispatchMessage(Handler.java:102)            at androID.os.Looper.loop(Looper.java:135)            at androID.app.ActivityThread.main(ActivityThread.java:5221)            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:899)            at com.androID.internal.os.ZygoteInit.main(ZygoteInit.java:694)

任何指针将不胜感激.

解决方法:

首先,我建议您使用XML文件来定义布局,而不是以编程方式编写它们(您可以搜索任何类型的示例和教程).

无论如何,您的具体问题与未初始化的变量msgDim有关.将onCreate()更改为：

@OverrIDeprotected voID onCreate(Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    init();    createMessage();    msgDim = new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);    r.addVIEw(message,msgDim);    setContentVIEw(r);}

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