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  1. import java.util.*;
  2. import java.lang.*;
  3. import java.io.*;
  4. import java.lang.reflect.*;
  5.  
  6. /* Name of the class has to be "Main" only if the class is public. */
  7. class Ideone
  8. {
  9. 	public static void main (String[] args) throws java.lang.Exception
  10. 	{
  11. 		doTricky1();
  12. 		doTricky2();
  13. 		System.out.printf(" 128 + 1 = %d\n",128+1);
  14. 	}
  15.  
  16. 	private static void doTricky1(){
  17. 		try{
  18. 			Class iCache=Integer.class.getDeclaredClasses()[0];
  19. 			Field f = iCache.getDeclaredField("cache");
  20. 			int low=-128;// low 保持不動
  21. 			int high=1000;//把 cache 從 -128~127 改為 -128~1000
  22.  
  23. 			Integer[] cacheArray=new Integer[high-low];
  24. 			for(int k = 0 , j=low ; k < cacheArray.length; k++)
  25. 				cacheArray[k] = new Integer(j++);
  26. 			setFinalStaticValue(f,cacheArray);
  27.  
  28. 			f = iCache.getDeclaredField("high");
  29. 			setFinalStaticValue(f,high);
  30.  
  31. 		}catch(Exception e){
  32. 			e.printStackTrace();
  33. 		}
  34. 	}
  35.  
  36. 	private static void doTricky2(){
  37. 		try{
  38. 			Class iCache=Integer.class.getDeclaredClasses()[0];
  39. 			Field f = iCache.getDeclaredField("cache");
  40. 			f.setAccessible(true);
  41. 			Integer[] cacheArray=(Integer[])f.get(iCache);
  42. 			cacheArray[257]=5; // 128 + 129 = 257
  43.  
  44. 		}catch(Exception e){
  45. 			e.printStackTrace();
  46. 		}
  47. 	}
  48.  
  49. 	private static void setFinalStaticValue(Field fd,Object val){
  50. 		try{
  51. 			fd.setAccessible(true);
  52. 			Field mdfField = Field.class.getDeclaredField("modifiers");
  53. 			mdfField.setAccessible(true);
  54. 			//重點是這裡，把 final 屬性拿掉
  55. 			mdfField.setInt(fd, fd.getModifiers() & ~Modifier.FINAL);
  56. 			fd.set(null, val);
  57. 		}catch(Exception e){
  58. 			e.printStackTrace();
  59. 		}
  60. 	}
  61. }
Success #stdin #stdout 0.08s 380224KB
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Standard input is empty
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 128 + 1 = 5
https://ideone.com/XO1zGU
language:
Java (HotSpot 12)
created:
11 years ago
visibility:
public

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