Java：向量的大小（以字節為單位）

Question

是否有可能在java中以字節為單位獲取Vector或ArrayList或任何對象的大小？ size（）函數只給出元素的數量。 但我想得到對象的實際大小。

Answer 1

目前，我實現了自己的規模。

long sizeof(ArrayList<String> list)
{
    long size = 0;
    for(String s: list)
        size+=s.length();
    return size;
}

Answer 2

不同的JDK對於隱含對象具有不同的大小。

您可以通過隱含對象大小乘以向量或arraylist的長度來計算它。 例如，如果聲明一個包含10條記錄的整數的araylist，則Java整數為32位，即32/8 = 4字節。 你有10條記錄，因此10x4 = 40 byetes。

標准java字節= 8位，短= 16位，int = 32位，長= 64位。

Answer 3

第一：嘗試閱讀Java中是否有像“sizeof（）”這樣的運算符

第二： 試試這段代碼 ：

public class Sizeof
{
    public static void main (String [] args) throws Exception
    {
        // Warm up all classes/methods we will use
        runGC ();
        usedMemory ();
        // Array to keep strong references to allocated objects
        final int count = 100000;
        Object [] objects = new Object [count];

        long heap1 = 0;
        // Allocate count+1 objects, discard the first one
        for (int i = -1; i < count; ++ i)
        {
            Object object = null;

            // Instantiate your data here and assign it to object

            object = new Object ();
            //object = new Integer (i);
            //object = new Long (i);
            //object = new String ();
            //object = new byte [128][3]

            if (i >= 0)
                objects [i] = object;
            else
            {
                object = null; // Discard the warm up object
                runGC ();
                heap1 = usedMemory (); // Take a before heap snapshot
            }
        }
        runGC ();
        long heap2 = usedMemory (); // Take an after heap snapshot:

        final int size = Math.round (((float)(heap2 - heap1))/count);
        System.out.println ("'before' heap: " + heap1 +
                            ", 'after' heap: " + heap2);
        System.out.println ("heap delta: " + (heap2 - heap1) +
            ", {" + objects [0].getClass () + "} size = " + size + " bytes");
        for (int i = 0; i < count; ++ i) objects [i] = null;
        objects = null;
    }
    private static void runGC () throws Exception
    {
        // It helps to call Runtime.gc()
        // using several method calls:
        for (int r = 0; r < 4; ++ r) _runGC ();
    }
    private static void _runGC () throws Exception
    {
        long usedMem1 = usedMemory (), usedMem2 = Long.MAX_VALUE;
        for (int i = 0; (usedMem1 < usedMem2) && (i < 500); ++ i)
        {
            s_runtime.runFinalization ();
            s_runtime.gc ();
            Thread.currentThread ().yield ();

            usedMem2 = usedMem1;
            usedMem1 = usedMemory ();
        }
    }
    private static long usedMemory ()
    {
        return s_runtime.totalMemory () - s_runtime.freeMemory ();
    }

    private static final Runtime s_runtime = Runtime.getRuntime ();
} // End of class

Answer 4

這應該顯示內存使用情況

Runtime rt = Runtime.getRuntime();
long m0 = rt.totalMemory() -  rt.freeMemory();
Object obj = new Object();      // create your object here
long m1 = rt.totalMemory() -  rt.freeMemory();
System.out.println(m1 - m0);

但它不起作用。 這有效（至少在我的電腦上）

public static void main(String[] args) throws Exception {
    Object obj = createObject();
    long m0 = usedMem();
    long m1 = usedMem();
    obj = null;
    for (int i = 0; i < 100; i++)
        System.gc();
    m0 = usedMem();
    obj = createObject();
    for (int i = 0; i < 100; i++)
        System.gc();
    m1 = usedMem();
    System.out.println(m1 - m0);
}

private static long usedMem() {
    return Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();
}

private static Object createObject() {
    return new Object();
}

如果它為新的Object打印8 ，為新的String（）打印40 ，那么它可以正常工作，然后創建您的對象並查看其大小

在這里查看更多http://www.javaspecialists.eu/archive/Issue029.html