Анбоксинг в современной Java

Java . : var Java 10, switch- Java 14, Java 16. , , , -, . , , , Java 14 - for . , , , Java.

for:

for (int i = 0; i < 10; i++) {
  System.out.println(i);
}
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

. -, . . -, effectively final. . : . . , - , , , - . , . .

, . , :

for (x in 0 until 10) {
  println(x)
}
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

: , , , . .

Java? , for-each, Java 5. :

/**
 *    
 * @param fromInclusive   ()
 * @param toExclusive   ( )
 * @return Iterable,    fromInclusive  toExclusive.
 */
public static Iterable<Integer> range(int fromInclusive, 
                                      int toExclusive) {
  return () -> new Iterator<Integer>() {
    int cursor = fromInclusive;
    public boolean hasNext() { return cursor < toExclusive; }
    public Integer next() { return cursor++; }
  };
}
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

rocket science, , . Java:

for (int i : range(0, 10)) { //    
  System.out.println(i);
}
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

. final, . . ? - . JMH-:

@Param({"1000"})
private int size;

@Benchmark
public int plainFor() {
  int result = 0;
  for (int i = 0; i < size; i++) {
    result += i * i * i;
  }
  return result;
}

@Benchmark
public int rangeFor() {
  int result = 0;
  for (int i : range(0, size)) {
    result += i * i * i;
  }
  return result;
}
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

, - , JIT- . , JIT . Java 8 :

Benchmark            (size)  Mode  Cnt     Score     Error  Units 
BoxedRange.plainFor    1000  avgt   30   622.679 ±   7.286  ns/op 
BoxedRange.rangeFor    1000  avgt   30  3591.052 ± 792.159  ns/op
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

range : 3,5 0,6 . -prof gc



, , rangeFor 13952 , plainFor . , , , 127 . Integer 128-999, 872 16 . , , Iterable, Iterator : (scalar replacement). .

, for , Java . Java:

: Java 14 range ! JIT- , , .

. Java 8 JVM -XX:+UnlockExperimentalVMOptions -XX:+AggressiveUnboxing



. , , :

Java 8-11 0,9 , 12 0,8, 13 . Java 14 , . , . , , .

? - Integer 127. valueOf ( Java 16):

public static Integer valueOf(int i) {
  if (i >= IntegerCache.low && i <= IntegerCache.high)
    return IntegerCache.cache[i + (-IntegerCache.low)];
  return new Integer(i);
}
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

, IntegerCache.low IntegerCache.high , . , , : . AggressiveUnboxing JIT- , , . , - :

Field field = Class.forName("java.lang.Integer$IntegerCache").getDeclaredField("cache");
field.setAccessible(true);
Integer[] arr = (Integer[]) field.get(null);
arr[130] = new Integer(1_000_000);
for (int i = 0; i < 10000; i++) {
  int res = rangeFor();
  if (res != -1094471800) {
    System.out.println("oops! " + res + "; i = " + i);
    break;
  }
}
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

, , . Java if



. AggressiveUnboxing

oops! 392146832; i = 333

JIT- C2 rangeFor, , , , .

, Java 12 cmp r10d,7fh



, 127 (=0x7f). , Java 13. , , , - . , Java 12 rangeFor 8 , Java 13 16 , plainFor.

, : Java 14, . for (int i : range(0, 10))



Java for (int i = 0; i < 10; i++)



, .

Valhalla. Iterable<int>



, . , range . Iterable<Integer>



.