list.ensureCapacity竟然会变慢

jdk1.8

应该是做了优化了；

public class Test10 {    public static void main(String[] args) {        ArrayList
     
       list = new ArrayList<>();        long beginTime = System.currentTimeMillis();        for (int i = 0; i < Integer.MAX_VALUE / 300; i++) {            list.add(i);        }        long endTime = System.currentTimeMillis();        CusPrint.print(endTime - beginTime); // 344    }}public class Test11 {    public static void main(String[] args) {        ArrayList
      
        list = new ArrayList<>();//        ArrayList
       
         list = new ArrayList<>(Integer.MAX_VALUE/300); // 这样也挺慢的        CusPrint.print("---");        list.ensureCapacity(Integer.MAX_VALUE/300); // 这样竟然比不加这句要慢        CusPrint.print("---");        long beginTime = System.currentTimeMillis();        for (int i = 0; i < Integer.MAX_VALUE / 300; i++) {            list.add(i);        }        long endTime = System.currentTimeMillis();        CusPrint.print(endTime - beginTime);  // 3391    }}
       
      
     

 测了一下还是数组快：

beginTime2 = System.currentTimeMillis();        int [] arr1 = new int[Integer.MAX_VALUE/300];        for(int i=0;i

 放到一起，修改一下list的最大容量，数组就是快，使用了ensureCapacity就是慢；

public class Test11_2 {    public static void main(String[] args) {        ArrayList
     
       list = new ArrayList<>();        CusPrint.print("---");        int capacity = Integer.MAX_VALUE/30;        list.ensureCapacity(capacity); // 这样竟然比不加这句要慢        CusPrint.print("---");        long beginTime = System.currentTimeMillis();        for (int i = 0; i < capacity; i++) {            list.add(i);        }        long endTime = System.currentTimeMillis();        CusPrint.print(endTime - beginTime);  // 26524        CusPrint.print("---------------");        ArrayList
      
        list2 = new ArrayList<>();        long beginTime2 = System.currentTimeMillis();        for (int i = 0; i < capacity; i++) {            list2.add(i);        }        long endTime2 = System.currentTimeMillis();        CusPrint.print(endTime2 - beginTime2); // 20126        CusPrint.print("---------------");        for(int i=0;i
      
     

 改到Integer.MAX_VALUE/3；直接就OOM了；

list.ensureCapacity(capacity);  // 就是这句就可以OOM

异常：

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space	at java.util.Arrays.copyOf(Arrays.java:3210)	at java.util.Arrays.copyOf(Arrays.java:3181)	at java.util.ArrayList.grow(ArrayList.java:265)	at java.util.ArrayList.ensureExplicitCapacity(ArrayList.java:239)	at java.util.ArrayList.ensureCapacity(ArrayList.java:219)	at com.stono.thread3.page_arraylist.Test11_2.main(Test11_2.java:14)

你在凝望深渊的时候，深渊也在凝望你，别看太久。

 哈哈，再看会儿：

使用list.trimToSize();竟然可以变快；

public class Test30 {    public static void main(String[] args) {        ArrayList list = new ArrayList();        list.ensureCapacity(Integer.MAX_VALUE / 300);        list.trimToSize();        long beginTime = System.currentTimeMillis();        for (int i = 0; i < Integer.MAX_VALUE / 300; i++) {            list.add(i);        }        long endTime = System.currentTimeMillis();        System.out.println(endTime - beginTime);    }}