要在Android應用中實現放大縮小功能，可以使用GestureDetector類和ScaleGestureDetector類來進行操作。以下是一個簡單的示例代碼：

1. 在你的Activity中實現GestureDetector.OnGestureListener和ScaleGestureDetector.OnScaleGestureListener接口：

public class MainActivity extends AppCompatActivity implements GestureDetector.OnGestureListener, ScaleGestureDetector.OnScaleGestureListener {
    private ScaleGestureDetector mScaleGestureDetector;
    private float mScaleFactor = 1.0f;

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

        mScaleGestureDetector = new ScaleGestureDetector(this, this);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        mScaleGestureDetector.onTouchEvent(event);
        return true;
    }

    @Override
    public boolean onScale(ScaleGestureDetector detector) {
        mScaleFactor *= detector.getScaleFactor();
        mScaleFactor = Math.max(0.1f, Math.min(mScaleFactor, 5.0f)); //限制縮放范圍
        //對視圖進行縮放操作
        yourView.setScaleX(mScaleFactor);
        yourView.setScaleY(mScaleFactor);
        return true;
    }

    @Override
    public boolean onScaleBegin(ScaleGestureDetector detector) {
        return true;
    }

    @Override
    public void onScaleEnd(ScaleGestureDetector detector) {

    }

    //其他手勢操作實現
}

2. 在這段代碼中，我們實現了onScale()方法來處理縮放操作，我們通過ScaleGestureDetector獲取縮放系數并在視圖上進行縮放。在onTouchEvent()方法中調用ScaleGestureDetector的onTouchEvent()方法來處理觸摸事件。

3. 請注意，這只是一個簡單的示例代碼，實際應用中可能需要根據具體的需求來進行調整和優化。