Android 多重线性布局不工作

两个

LinearLayout

linearfirstRow

和

linearsecondRow

都有四个按钮。第一个线性布局工作，第二个布局不显示

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linearfirstrow

正在显示，因为此

setContentView（linearfirstrow）

要显示linearsecondRow，请在
onCreate（）的最后一部分添加此项

为什么不通过XML来创建UI？
protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        linearfirstRow=new LinearLayout(this);
        linearfirstRow.setOrientation(LinearLayout.HORIZONTAL);
        linearfirstRow.setId(1);
        LinearLayout.LayoutParams Layoutparam1 = new LinearLayout.LayoutParams(
                LayoutParams.WRAP_CONTENT,
                LayoutParams.WRAP_CONTENT,25);      
        linearfirstRow.setLayoutParams(Layoutparam1);
        setContentView(linearfirstRow);


        linearsecondRow=new LinearLayout(this);
        linearsecondRow.setOrientation(LinearLayout.HORIZONTAL);
        linearsecondRow.setId(2);   
        linearsecondRow.setLayoutParams(Layoutparam1);


        btn_1=new Button(this);
        btn_1.setLayoutParams(Layoutparam1);
        btn_2=new Button(this);
        btn_2.setLayoutParams(Layoutparam1);
        btn_3=new Button(this);
        btn_3.setLayoutParams(Layoutparam1);
        btn_4=new Button(this);
        btn_4.setLayoutParams(Layoutparam1);

        btn_5=new Button(this);
        btn_5.setLayoutParams(Layoutparam1);
        btn_6=new Button(this);
        btn_6.setLayoutParams(Layoutparam1);
        btn_7=new Button(this);
        btn_7.setLayoutParams(Layoutparam1);
        btn_8=new Button(this);
        btn_8.setLayoutParams(Layoutparam1);

        linearfirstRow.addView(btn_1);
        linearfirstRow.addView(btn_2);
        linearfirstRow.addView(btn_3);
        linearfirstRow.addView(btn_4);

buttons are -->     linearsecondRow.addView(btn_5);
notdisplayed        linearsecondRow.addView(btn_6);
                    linearsecondRow.addView(btn_7);
                    linearsecondRow.addView(btn_8);

addView(linearsecondRow);