How to use onDraw/drawRect to make a rectangle based on editText from the mainActivity

Question

Basically I am trying to gather the size of room from the user..(example 20x20 size room). Using those dimensions, collected on the mainActivity, send that data to the Result Activity and use the onDraw/onMeasure to create a simple box.

In short, I want to collect the size of the box in mainActivity from the user. Then in Result activity show the completed image of the box.

I have completed the passing Intents with data from one activity to another. Its using that collected data to draw a shape I am having trouble with.

public class MainActivity extends AppCompatActivity {

    //variables
    Button btnCalculate, btnReset;
    Spinner spinSpeaker;
    EditText editLength, editWidth, editHeight;
    String t1,t2,t3;

    int numLength, numWidth, numHeight, numResult=0;


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

        //connecting variables to GUI elements
        btnCalculate= findViewById(R.id.btnCalculate);
        btnReset = findViewById(R.id.btnReset2);
        spinSpeaker = findViewById(R.id.spinSpeaker);
        editLength= findViewById(R.id.editLength);
        editWidth = findViewById(R.id.editWidth);
        editHeight = findViewById(R.id.editHeight);




        //adding function to the calculate button
        btnCalculate.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                //parsing text to integers
                t1=editLength.getText().toString();
                t2=editWidth.getText().toString();
                t3=editHeight.getText().toString();

                if (t3.equals("")) {
                    editHeight.setText("11");
                }else if (t2.equals("")) {
                    editWidth.setText("11");
                }else if (t1.equals("")) {
                    editLength.setText("11");
                }else{
                    numLength = Integer.parseInt(t1);
                    numWidth = Integer.parseInt(t2);
                    numHeight = Integer.parseInt(t3);
                }

                //alert to contact Pro design
                if ((numLength>=999)||(numWidth>=999)||(numHeight>=20)) {

                    Toast.makeText(MainActivity.this, R.string.contactPro,
                            Toast.LENGTH_LONG).show();
                }else {
                    //calculating volume
                    numResult = numHeight * numWidth * numLength;

                    //sending data to other activities
                    Intent intent = new Intent(MainActivity.this, Result.class);
                    intent.putExtra("varHeight", numHeight);
                    intent.putExtra("varWidth", numWidth);
                    intent.putExtra("varLength", numLength);
                    intent.putExtra("varResult", numResult);
                    startActivity(intent);
                }
            }
        });


    }

//Result activity
public class Result extends AppCompatActivity {

    TextView tim;
    MyCanvas mycanvas;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        mycanvas = new MyCanvas(this);
        setContentView(R.layout.activity_result);
        setContentView(Jim);


        //Pulling variables from main activity
        Intent variables = getIntent();
        //int length = variables.getIntExtra("varLength",0);
        //int width =variables.getIntExtra("varWidth",0);
        int totalResult = variables.getIntExtra("varResult",0);
        //showing total
        String result = String.valueOf(totalResult);
        tim = findViewById(R.id.txtResult);
        tim.setText(result);


        //Calling the reset button method
        configureResetBtn();

    }

        public void configureResetBtn(){
    Button btnReset=findViewById(R.id.btnReset2);
    btnReset.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            //startActivity(new Intent(Result.this, MainActivity.class));
            finish();
        }
    });
}


}

//MyCanvas activity
public class MyCanvas extends AppCompatActivity {


    public MyCanvas(Result result) {
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Intent variables = getIntent();
        final int length = variables.getIntExtra("varLength", 0);
        final int width = variables.getIntExtra("varWidth", 0);


         class Jim extends View {

            Paint paint = new Paint();
            Rect rect = new Rect();

            public Jim(Context context) {
                super(context);
            }

            @Override
            protected void onDraw(android.graphics.Canvas canvas) {
                super.onDraw(canvas);
                paint.setStrokeWidth(3);

                canvas.drawRect(0, 0, canvas.getWidth(), canvas.getHeight(), paint);
            }

            @Override
            protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
                super.onMeasure(widthMeasureSpec, heightMeasureSpec);

                setMeasuredDimension(length,width);
            }
        }
    }
}

The expected result is to show a box that is the dimensions based on the user (example 20x20). However I am getting this error:

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

But my button works when I remove code trying to draw the box.

Answer 1

Instead of setting content view twice In ResultActivity use SurfaceView in R.layout.activity_result

<FrameLayout 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" >

    <SurfaceView
        android:id="@+id/surface_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
//....

</FrameLayout>

In ResultActivity

SurfaceView surfaceView = findViewById(R.id.surface_view);

surfaceView.getHolder().addCallback(new Callback() {
    @Override
    public void surfaceCreated(SurfaceHolder holder) {

        Canvas canvas = holder.lockCanvas();
         // draw
        Paint paint = new Paint();
        paint.setStrokeWidth(3);
        canvas.drawRect(0, 0, canvas.getWidth(), canvas.getHeight(), paint);

        holder.unlockCanvasAndPost(canvas);
    }

    @Override
    public void surfaceDestroyed(SurfaceHolder holder) {
    }

    @Override
    public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
    }
});