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  1. import java.util.HashMap;
  2. import java.util.Map;
  3.  
  4. public final class Main {
  5.  
  6.     private final Map<Character, Runnable> map;
  7.  
  8.     public Main() {
  9.         map = new HashMap<>();
  10.         map.put('h', () -> moveCursorLeft());
  11.         map.put('j', () -> moveCursorDown());
  12.         map.put('k', () -> moveCursorUp());
  13.         map.put('l', () -> moveCursorRight());
  14.     }
  15.  
  16.     public void handleKeyInput() {
  17.         var c = getPressedKey();
  18.         var action = map.get(c);
  19.         // Since 'action' can be null, the next statement may throw
  20.         // a NullPointerException.
  21.         action.run();
  22.     }
  23.  
  24.     /**
  25.         Returns the character of the pressed key.
  26.         If no key is pressed, blocks until any key is pressed.
  27.  
  28.         @return
  29.             The character of the pressed key.
  30.     */
  31.     private char getPressedKey() {
  32.         return 'a';
  33.     }
  34.  
  35.     private void moveCursorLeft() {}
  36.     private void moveCursorDown() {}
  37.     private void moveCursorUp() {}
  38.     private void moveCursorRight() {}
  39.  
  40.     public static void main(String[] args) {
  41.         var m = new Main();
  42.         m.handleKeyInput();
  43.     }
  44. }
  45.  
Runtime error #stdin #stdout #stderr 0.06s 33608KB
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stdin
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Standard input is empty
stdout
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Standard output is empty
stderr
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Exception in thread "main" java.lang.NullPointerException
	at Main.handleKeyInput(Main.java:21)
	at Main.main(Main.java:42)
https://ideone.com/lgH2yN
The Insanity Beyond Null: Null object pattern #1
language:
Java (HotSpot 12)
created:
4 years ago
visibility:
public

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