在方法范围外使用变量

设置成全局的话
private String char1,char2;
 @Override
 protected void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
 setContentView(R.layout.activity_pick_char2);
 Bundle extras = getIntent().getExtras();
 if (extras != null) {
 String char1 = (String) extras.getString("char1"); 
 //上面修改为char1 = (String) extras.getString("char1");
 TextView textView = (TextView) findViewById(R.id.header);
}


}

textView.setText(char1+" vs "+"char2");//这句话你就得等char1 char2都赋值完成后才能设置了

public void selectChar2(View v) {
 Button btn = (Button)v;
 String char2 = btn.getText().toString();//这个也得把String去掉