无法从MainActivity.java创建布局

Question

I m trying to create a layout from MainActivity.java but my emulator is not showing the layout and the button which i have created from MainActivity.java. Do i have to change something in activity_main.xml? Here is the code,

package com.example.pratt.myapplication;

import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.util.Log;
import android.widget.RelativeLayout;
import android.widget.Button;
import android.graphics.Color;


public class MainActivity extends ActionBarActivity {

    private static final String TAG="My Message";

@Override
    protected void onCreate(Bundle savedInstanceState) {
        /*
        */
        super.onCreate(savedInstanceState);
        RelativeLayout myLayout=new RelativeLayout(this);
        Button blue_button=new Button(this);
        myLayout.setBackgroundColor(Color.RED);
        myLayout.addView(blue_button);
         blue_button.setText("Blue Button");
        Log.i(TAG,"onCreate");
        setContentView(myLayout);
    }

    @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);
    }
}

Answer 1

I didn't test your code, but I guess it's because you didn't to add some LayoutParams for your layout, for example, your layout's width and height. If you don't want to use fixed width and height you can use LayoutParams.WRAP_CONTNT or MATCH_PARENT instead.

Answer 2

I suppose you should create the Button before @Override .

private static final String TAG="My Message";

@Override
    protected void onCreate(Bundle savedInstanceState) {
        /*
        */
        super.onCreate(savedInstanceState);
        RelativeLayout myLayout=new RelativeLayout(this);
        myLayout.setBackgroundColor(Color.RED);
        Button blue_button=new Button(this);
        blue_button.setLayoutParams(new LayoutParams(LayoutParams.WRAP_CONTENT,       
        LayoutParams.WRAP_CONTENT)
        myLayout.addView(blue_button);
        blue_button.setText("Blue Button");
        Log.i(TAG,"onCreate");
        setContentView(myLayout);
    }

You have not specified the dimensions of the button . Hence, it creates a button of 0dp x 0dp .

Hope this helps !