Null Pointer Exception when setting LayoutParams

Question

I created button in activity programmaticaly, not in xml file. Then I wanted to set it LayoutParams like in this link: How to programmatically set the layout_align_parent_right attribute of a Button in Relative Layout?

But when I tried to launch, I had an exception.

Here is my code:

RelativeLayout ll2 = new RelativeLayout(this);
    //ll2.setOrientation(LinearLayout.HORIZONTAL);

    ImageButton go = new ImageButton(this);
    go.setId(cursor.getInt(cursor.getColumnIndex("_id")));
    go.setClickable(true);
    go.setBackgroundResource(R.drawable.go);
    RelativeLayout.LayoutParams params1 = (RelativeLayout.LayoutParams)go.getLayoutParams();
    params1.addRule(RelativeLayout.ALIGN_PARENT_RIGHT); // LogCat said I have Null Pointer Exception in this line
    go.setLayoutParams(params1);
    go.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent i3 = new Intent(RecipesActivity.this, MedicineActivity.class);
            i3.putExtra(ShowRecipe, v.getId());
            i3.putExtra("Activity", "RecipesActivity");
            RecipesActivity.this.startActivity(i3);
        }
    });
    ll2.addView(go);

Why my app throws an exception? Thanks.

Answer 1

Change this:

RelativeLayout.LayoutParams params1 = (RelativeLayout.LayoutParams)go.getLayoutParams();
params1.addRule(RelativeLayout.ALIGN_PARENT_RIGHT);
go.setLayoutParams(params1);

for:

RelativeLayout.LayoutParams params1 = new RelativeLayout.LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT);
params1.addRule(RelativeLayout.ALIGN_PARENT_RIGHT);
go.setLayoutParams(params1);

When you create a View programatically it doesn't have any LayoutParams, that's why you are getting NullPointerException. If you inflate the view from XML, the view is coming now with LayoutParams.

Answer 2

From what I see, it looks like

go.getLayoutParams()

returns null in this line

 RelativeLayout.LayoutParams params1 = (RelativeLayout.LayoutParams)go.getLayoutParams();

because you have no LayoutParams set for it. This will obviously make params1 null so you get NPE here

 params1.addRule(RelativeLayout.ALIGN_PARENT_RIGHT);

when you try to run a method on it (addRule(RelativeLayout.ALIGN_PARENT_RIGHT))

getLayoutParams() is for a View that has params already set. So you will need to set them for go before trying to get them to use for another View

Answer 3

This method may return null if this View is not attached to a parent ViewGroup or {@link #setLayoutParams(android.view.ViewGroup.LayoutParams)} was not invoked successfully. When a View is attached to a parent ViewGroup, this method must not return null.

According to the annotation of getLayoutParams() in the View.java, your ImageButton is created programly and is not attached to a parent ViewGroup when you call the method, so it returns null.