list.ensureCapacity竟然会变慢

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<Integer> 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<Integer> list = new ArrayList<>();
//        ArrayList<Integer> 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<Integer.MAX_VALUE/300;i++) {
            arr1[i] = i;
        }
        endTime2 = System.currentTimeMillis();
        CusPrint.print(endTime2 - beginTime2); // 59

 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<Integer> 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<Integer> 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<capacity;i++) {
            if(i<capacity) {
                
            }
        }
        CusPrint.print("---------------");
        for(int i=0;i<capacity;i++) {
            if(i<i+1) {
                
            }
        }
        CusPrint.print("---------------");
        beginTime2 = System.currentTimeMillis();
        int [] arr1 = new int[capacity];
        for(int i=0;i<capacity;i++) {
            arr1[i] = i;
        }
        endTime2 = System.currentTimeMillis();
        CusPrint.print(endTime2 - beginTime2); // 47
        CusPrint.print("---------------");
        
    }

}

 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);
    }
}