Your first Android app, from the ground up

Understand activities, connect a layout, and make your first button do something.

1. Start with a Views project

Create a Java-based Empty Views Activity in Android Studio. This lesson uses platform Activity to keep the example focused. If your generated project uses AppCompat, retain its dependency and theme or adapt the superclass deliberately.

The example package is com.example.counter. Match your project's namespace, or update the package declaration. Use an installed Android SDK and keep your generated Gradle configuration.

2. Describe the screen

Open res/layout/activity_main.xml. Give the TextView and Button unique IDs. A vertical LinearLayout places them one above the other; gravity="center" centers the group.

Use dp for padding and sp for text size so the interface responds to density and the user's font settings.

3. Connect the activity

Call setContentView before findViewById. The activity loads the XML, finds each view, and registers a click listener. Convert the integer to a string before calling setText; the integer overload expects a string resource ID.

4. Preserve the counter

The activity reads the saved count in onCreate and writes it in onSaveInstanceState. This handles activity recreation such as a device rotation. It is not permanent storage: use a database or preferences for data that must persist between sessions.

Expected output

The screen starts at 0. Each tap on Add one increments the visible number. Rotating the device keeps the current value when Android restores the activity state.

Troubleshooting

• Cannot resolve R: check XML errors and the module namespace first.
• NullPointerException: confirm you loaded the correct layout and the IDs match.
• Resources.NotFoundException: pass String.valueOf(count) to setText, rather than the integer itself.
• App does not appear in launcher: check the exported launcher activity in the manifest.

Try it yourself

Add a reset button. Set the counter to zero and update the TextView in its click listener. Then test rotation and confirm both buttons still work.

package com.example.counter;

import android.app.Activity;
import android.os.Bundle;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends Activity {
    private int count = 0;

    @Override
    protected void onCreate(Bundle state) {
        super.onCreate(state);
        setContentView(R.layout.activity_main);
        if (state != null) count = state.getInt("count", 0);
        TextView label = findViewById(R.id.counter);
        Button button = findViewById(R.id.increment);
        label.setText(String.valueOf(count));
        button.setOnClickListener(view -> {
            count++;
            label.setText(String.valueOf(count));
        });
    }

    @Override
    protected void onSaveInstanceState(Bundle state) {
        state.putInt("count", count);
        super.onSaveInstanceState(state);
    }
}

Keep connecting the dots