安卓基础 求大神指点！

为啥这地方总是报错啊 求大神指点！

public class MainActivity extends AppCompatActivity implements View.OnClickListener {
private Button button1,button2,button3,button4;
private LinearLayout linearLayout;
private Fragment1 fragment1;
private Fragment2 fragment2;
private Fragment3 fragment3;
private Fragment4 fragment4;
private FragmentManager fragmentManager;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    button1=findViewById(R.id.bt1);
    button2=findViewById(R.id.bt2);
    button3=findViewById(R.id.bt3);
    button4=findViewById(R.id.bt4);
    fragment1=new Fragment1();
    fragment2=new Fragment2();
    fragment3=new Fragment3();
    fragment4=new Fragment4();
    fragmentManager=getSupportFragmentManager();
    button1.setOnClickListener(this);
    button2.setOnClickListener(this);
    button3.setOnClickListener(this);
    button4.setOnClickListener(this);

// button1.setOnClickListener(new View.OnClickListener() {
// @Override
// public void onClick(View v) {
// Fragment1 fragment1=new Fragment1();
// FragmentManager fragmentManager=getSupportFragmentManager();
// FragmentTransaction fragmentTransaction=fragmentManager.beginTransaction();
// fragmentTransaction.replace(R.id.ll,fragment1);
// fragmentTransaction.commit();
// }
// });
// button2.setOnClickListener(new View.OnClickListener() {
// @Override
// public void onClick(View v) {
// Fragment1 fragment1=new Fragment1();
// FragmentManager fragmentManager=getSupportFragmentManager();
// FragmentTransaction fragmentTransaction=fragmentManager.beginTransaction();
// fragmentTransaction.replace(R.id.ll,fragment1);
// fragmentTransaction.commit();
// }
// });

}


@Override
public void onClick(View v) {
    switch (v.getId())
    {
        case R.id.bt1:
            clickBt1();
            break;
        case R.id.bt2:
            clickBt2();
            break;
        case R.id.bt3:
            clickBt3();
            break;
        case R.id.bt4:
            clickBt4();
            break;

    }
}
public void clickBt1()
{
    FragmentTransaction fragmentTransaction=fragmentManager.beginTransaction();
    fragmentTransaction.replace(R.id.ll,fragment1);
    fragmentTransaction.commit();
    button1.setTextColor(Color.RED);
    button2.setTextColor(Color.BLACK);
    button3.setTextColor(Color.BLACK);
    button4.setTextColor(Color.BLACK);


}
public void clickBt2()
{
    FragmentTransaction fragmentTransaction=fragmentManager.beginTransaction();
    fragmentTransaction.replace(R.id.ll,fragment2);
    fragmentTransaction.commit();
    button2.setTextColor(Color.BLUE);
    button1.setTextColor(Color.BLACK);
    button3.setTextColor(Color.BLACK);
    button4.setTextColor(Color.BLACK);

}
public void clickBt3()
{
    FragmentTransaction fragmentTransaction=fragmentManager.beginTransaction();
    fragmentTransaction.replace(R.id.ll,fragment3);
    fragmentTransaction.commit();
    button3.setTextColor(Color.RED);
    button2.setTextColor(Color.BLACK);
    button1.setTextColor(Color.BLACK);
    button4.setTextColor(Color.BLACK);


}
public void clickBt4()
{
    FragmentTransaction fragmentTransaction=fragmentManager.beginTransaction();
    fragmentTransaction.replace(R.id.ll,fragment4);
    fragmentTransaction.commit();
    button4.setTextColor(Color.RED);
    button2.setTextColor(Color.BLACK);
    button3.setTextColor(Color.BLACK);
    button1.setTextColor(Color.BLACK);


}

}

onCreate里面缺少setContentView。

你的布局呢？ 不添加布局进去，怎么识别控件id.

没有setContentView方法 你怎么findviewbyId =_=