Error creating Relative Layout using Java Code

Question

i am new to android development and was trying to create RelativeLayout using Java code. Below is my 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;

@Override
protected 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));

}

}

However i get the null pointer exception(below from Logcat)

 Process: com.rt.droid.rellayoutjava, PID: 2471
java.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.access$800(ActivityThread.java:144)
        at android.app.ActivityThread$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.access$800(ActivityThread.java:144)
            at android.app.ActivityThread$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)

Any pointers would be appreciated.

Answer 1

First of all, I suggest you to use XML files to define your layout, instead of writing them programmatically (you can search for any kind of example and tutorial).

Anyway, your specific problem is related to the variable msgDim which is not initialized. Change your onCreate() to this:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    init();
    createMessage();
    msgDim = new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
    r.addView(message,msgDim);
    setContentView(r);
}