[問題] CallBack Listener 問題請教

看板java作者lihohsin (新)時間3年前 (2021/12/03 19:21)推噓1(1推 0噓 16→)

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小弟在學習callback listener 參考這個 https://guides.codepath.com/android/Creating-Custom-Listeners#3-implement-listener-callback 練習的代碼如下 MainActivity2.java public class MainActivity2 extends AppCompatActivity { Button button2; TextView textView2; DataListener listener; public MainActivity2(){ this.listener=null; } @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main2); button2=findViewById(R.id.button2); textView2=findViewById(R.id.textView2); textView2.setText(String.valueOf(listener.getData())); } public interface DataListener{ int getData(); } public void setDataListener(DataListener listener){ this.listener = listener; } } MainActivity.java public class MainActivity extends AppCompatActivity { Button button; private int x=5; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); button =findViewById(R.id.button); button.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View view) { Intent i =new Intent(); i.setClass(MainActivity.this,MainActivity2.class); startActivity(i); finish(); } }); MainActivity2 mainActivity2=new MainActivity2(); mainActivity2.setDataListener(new MainActivity2.DataListener() { @Override public int getData() { return x; } }); } } 目標只是想要讓MainActivity2抓到 MainActivity中的x=5 可是執行之後 Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'int bestfood.today.callbackdemo.MainActivity2$DataListner.getData()' on a null object reference 總是報這行錯誤常是幾種寫法都報這個錯誤有沒有高手知道這個callback listener是哪裡寫錯了? -- ※ 發信站: 批踢踢實業坊(ptt.cc), 來自: 114.34.60.240 (臺灣) ※ 文章網址: https://www.ptt.cc/bbs/java/M.1638530491.A.94A.html

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12/04 01:36, 3年前 , 1^F

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12/04 01:37, 3年前 , 3^F

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