How to show imageView full screen on imageView click?

You can use ImageView below two properties to show image based on your requirement :

1. android:adjustViewBounds : Set this to true if you want the ImageView to adjust its bounds to preserve the aspect ratio of its drawable.

2. android:scaleType :Controls how the image should be resized or moved to match the size of this ImageView

   <ImageView
       android:id="@+id/imageView"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:adjustViewBounds="true"
       android:src="https://stackoverflow.com/questions/24463691/@drawable/ic_launcher"/>
   

Above two properties can be use either xml or java code.

As you need to decide at run time need to show image into full screen or not so will apply above two properties at java code as below :

public class MainActivity extends Activity {

    ImageView imageView;

    boolean isImageFitToScreen;

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

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

        imageView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if(isImageFitToScreen) {
                    isImageFitToScreen=false;
                    imageView.setLayoutParams(new LinearLayout.LayoutParams(LinearLayout.LayoutParams.WRAP_CONTENT, LinearLayout.LayoutParams.WRAP_CONTENT));
                    imageView.setAdjustViewBounds(true);
                }else{
                    isImageFitToScreen=true;
                    imageView.setLayoutParams(new LinearLayout.LayoutParams(LinearLayout.LayoutParams.MATCH_PARENT, LinearLayout.LayoutParams.MATCH_PARENT));
                    imageView.setScaleType(ImageView.ScaleType.FIT_XY);
                }
            }
        });

    }
}