How to put layout inside another layout?

I want to put a layout inside another layout but when I do this it gives NullPointerException on relLayout.addView(squareLayout); line

for (int i = 0; i < 开发者_开发知识库6; i++) {
    for (int j = 0; j < 7; j++) {

        RelativeLayout relLayout = (RelativeLayout) findViewById(relIds[i][j]);
        relLayout.removeAllViews();

        RelativeLayout squareLayout = (RelativeLayout) findViewById(R.id.square);
        relLayout.addView(squareLayout);
    }
}

Please tell me what should I do?

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relLayout is not null while accessing removeAllViews() method. So either findViewById() or another thread would have set null to relLayout instance.

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improper use of relIds[][] without initialization could have also caused Null pointer exception