这种情况是正常的

private Button button;
private Button button1;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    button = (Button)findViewById(R.id.button);
    button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            SharedPreferences.Editor editor = getSharedPreferences("data", 0).edit();
            editor.putInt("yi", 1);
            editor.putString("er", "二");
            editor.apply();
            Toast.makeText(MainActivity.this, "complete", Toast.LENGTH_SHORT).show();
        }
    });

这样就会报空指针错

private Button button= (Button)findViewById(R.id.button);;
private Button button1;

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

    button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            SharedPreferences.Editor editor = getSharedPreferences("data", 0).edit();
            editor.putInt("yi", 1);
            editor.putString("er", "二");
            editor.apply();
            Toast.makeText(MainActivity.this, "complete", Toast.LENGTH_SHORT).show();
        }
    });

请问这两种的区别在哪里？

 private Button button= (Button)findViewById(R.id.button);;//第二种初始化在oncreate执行前，
 也就是
 setContentView(R.layout.activity_main);还没执行，布局都还没加载，肯定找不到R.id.button

同上；第二个你布局都没有加载就先给button赋内容啦；肯定会报空指针的。。

button没赋值当然会OOM啦

setContentView后再findViewById