Why 'hello world' not displaying in android emulator

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Java

Mobile

Android

In my app project my emulator appears without 'hello world'.
I am able to see the clock and other settings but not my app's 'hello world'.
Can anyone help me out?

<resources>

    <string name="app_name">My Application</string>
    <string name="hello_world">Hello world!</string>
    <string name="action_settings">Settings</string>
    <string name="title_activity_main">MainActivity</string>
</resources>

package com.example.rajendran.myapplication;


import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.app.Activity;
import android.support.v4.app.NavUtils;

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }


    @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;
    }


}

<relativelayout xmlns:android="http://schemas.android.com/apk/res/android">
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >

    <textview>
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"

        android:text="@string/hello_world"
        tools:context=".MainActivity" />

    </textview>
</relativelayout>