How do I make one image not go over another?

Alright? I have a code where I can move several images with the mouse but when dragging an image it goes over the others. I want to move it without going over it.

Follow the codes:

package br.com.example.teste8;

import android.app.Activity;
import android.os.Bundle;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.RelativeLayout;

public class MainActivity extends Activity implements View.OnTouchListener {

    private ImageView mImageView;

    private ViewGroup mRrootLayout;

    private int _xDelta;
    private int _yDelta;

    RelativeLayout.LayoutParams layoutParams;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mRrootLayout = (ViewGroup) findViewById(R.id.marco);

        mImageView = (ImageView) mRrootLayout.findViewById(R.id.imageView);

        RelativeLayout.LayoutParams layoutParams = new RelativeLayout.LayoutParams(50,50);
        layoutParams.leftMargin = 0;
        layoutParams.topMargin = 10;

        mImageView.bringToFront();
        mImageView.setLayoutParams(layoutParams);
        mImageView.setOnTouchListener(this);


        mImageView = (ImageView) mRrootLayout.findViewById(R.id.imageView2);

        RelativeLayout.LayoutParams layoutParams1 = new RelativeLayout.LayoutParams(50,50);
        layoutParams1.leftMargin = 50;
        layoutParams1.topMargin = 10;

        mImageView.setLayoutParams(layoutParams1);
        mImageView.setOnTouchListener(this);


        mImageView = (ImageView) mRrootLayout.findViewById(R.id.imageView3);

        RelativeLayout.LayoutParams layoutParams2 = new RelativeLayout.LayoutParams(50,50);
        layoutParams2.leftMargin = 100;
        layoutParams2.topMargin = 10;

        mImageView.setLayoutParams(layoutParams2);
        mImageView.setOnTouchListener(this);

    }


    public boolean onTouch(View view, MotionEvent event) {
        final int X = (int) event.getRawX();
        final int Y = (int) event.getRawY();
        switch (event.getAction() & MotionEvent.ACTION_MASK) {

        case MotionEvent.ACTION_UP:
            break;

        case MotionEvent.ACTION_MOVE:
            RelativeLayout.LayoutParams layoutParams = (RelativeLayout.LayoutParams) view.getLayoutParams();

            layoutParams.leftMargin = X - _xDelta;
            layoutParams.topMargin = Y - _yDelta;
            layoutParams.rightMargin = -50;
            layoutParams.bottomMargin = -50;
            view.setLayoutParams(layoutParams);
            break;
        }
        mRrootLayout.invalidate();
        return true;
    }
}

XML:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/marco"
     android:layout_width="fill_parent"
     android:layout_height="fill_parent"
    android:orientation="horizontal"
    android:clickable="true">


 <TextView
     android:id="@+id/txt"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
     android:text="@string/hello_world"
     android:gravity="top" />

<ImageView 

    android:id="@+id/imageView"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@drawable/border"
    android:contentDescription="@null"
    android:src="@drawable/ic_launcher" />

<ImageView
    android:id="@+id/imageView2"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@drawable/border"
    android:contentDescription="@null"
    android:src="@drawable/ic_launcher" />

<ImageView 
    android:id="@+id/imageView3"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginLeft="48dp"
    android:background="@drawable/border"
    android:contentDescription="@null"
    android:src="@drawable/ic_launcher" />

</RelativeLayout>