JVM学习分享-思考题

Posted 2020-11-13 deskzero

package zero.desk.stringconstantpool;

import org.junit.Test;

/**
 * @author Zero
 * @since 2019-09-17.
 * Description:
 * 当调用intern方法时，
 * 如果池已经包含此字符串（equals确定），则返回池中字符串。
 * 否则，将此String对象添加到池中并且返回该String对象的引用。
 */
public class StringConstantPool_Intern {

    @Test
    public void test1() {
        String a1 = "AA";//当调用intern方法时，如果池已经包含此字符串（equals确定），则返回池中字符串。否则，将此String对象添加到池中并且返回该String对象的引用。
        a1.intern();
    }
}

package zero.desk.stringconstantpool;

import org.junit.Test;

/**
 * @author Zero
 * @since 2019-09-17.
 * Description:
 * /Users/zero/Code/Myself/Project/zero-Project/jvm/target/classes/zero/desk/stringconstantpool
 * javap -verbose StringConstantPoolAA
 * cd ../stringconstantpool
 */
public class StringConstantPoolAA {

    @Test
    public void test1() {
        String a1 = "AA";//在常量池上创建常量AA
        String a2 = "AA";//直接返回已经存在的常量AA
        System.out.println(a1 == a2); //true
    }

}

package zero.desk.stringconstantpool;

import org.junit.Test;

/**
 * @author Zero
 * @since 2019-09-17.
 * Description:
 * 思考题
 */
public class StringConstantPoolAAAA {

    @Test
    public void test1() {
        String a1 = new String("AA");//在堆上创建常量AA
        String a11 = a1.intern();
        String a2 = "AA";//在常量池上创建常量AA
        System.out.println(a1 == a2); //false
        System.out.println(a1 == a11); //false
        System.out.println(a11 == a2); //true
    }

}

package zero.desk.stringconstantpool;

import org.junit.Test;

/**
 * @author Zero
 * @since 2019-09-17.
 * Description:
 * 思考题
 */
public class StringConstantPoolAABB {

    @Test
    public void test1() {
        String a1 = "AA"+"BB";//在常量池上创建常量AABB
        String a2 = "AABB";//在常量池上创建常量AABB
        System.out.println(a1 == a2); //true
    }

}

package zero.desk.stringconstantpool;

import org.junit.Test;

/**
 * @author Zero
 * @since 2019-09-17.
 * Description:
 * 思考题
 */
public class StringConstantPoolNAA {

    @Test
    public void test1() {
        String a1 = "AA";//在常量池上创建常量AA
        String a2 = new String("AA");//在堆上创建对象AA
        String a3 = a2.intern();//常量池上存在常量AA，直接返回常量池上常量AA，即a1
        System.out.println(a1 == a2); //false
        System.out.println(a1 == a3); //true
        System.out.println(a2 == a3); //false
        System.out.println(System.identityHashCode(a1));
        System.out.println(System.identityHashCode(a2));
    }

}

package zero.desk.stringconstantpool;

import org.junit.Test;

/**
 * @author Zero
 * @since 2019-09-17.
 * Description:
 * 思考题
 */
public class StringConstantPoolNAABB {

    @Test
    public void test1() {
        String a3 = "AA"+new String("BB");    //在堆上创建对象AABB
//        String a33 = a3.intern();
        String a333 = "AABB";//在常量池上创建常量AABB
        System.out.println(a333 == a3); //false
//
        System.out.println(System.identityHashCode(a3));
        System.out.println(System.identityHashCode(a333));
    }
}

package zero.desk.stringconstantpool;

import org.junit.Test;

/**
 * @author Zero
 * @since 2019-09-17.
 * Description:
 * 思考题
 */
public class StringConstantPoolNAABBi {

    @Test
    public void test1() {
        String a3 = "AA"+new String("BB");    //在常量池上创建常量AABB
        String a33 = a3.intern();
        String a333 = "AABB";//在常量池上创建常量AABB
        System.out.println(a333 == a3); //true
//
        System.out.println(System.identityHashCode(a3));
        System.out.println(System.identityHashCode(a33));
        System.out.println(System.identityHashCode(a333));
    }
}

package zero.desk.stringconstantpool;

import org.junit.Test;

/**
 * @author Zero
 * @since 2019-09-17.
 * Description:
 * 思考题
 */
public class StringConstantPoolNAANBB {

    @Test
    public void test1() {
        String a3 = new String("AA")+new String("BB");    //在堆上创建对象AABB
//        String a33 = a3.intern();
        String a333 = "AABB";//在常量池上创建常量AABB
        System.out.println(a333 == a3); //false
//
        System.out.println(System.identityHashCode(a3));
        System.out.println(System.identityHashCode(a333));
    }
}

package zero.desk.stringconstantpool;

import org.junit.Test;

/**
 * @author Zero
 * @since 2019-09-17.
 * Description:
 * 思考题
 */
public class StringConstantPoolNAANBBi {

    @Test
    public void test1() {
        String a3 = new String("AA")+new String("BB");    //在常量池上创建常量AABB
        String a33 = a3.intern();
        String a333 = "AABB";//在常量池上创建常量AABB
        System.out.println(a333 == a3); //true
//
        System.out.println(System.identityHashCode(a3));
        System.out.println(System.identityHashCode(a33));
        System.out.println(System.identityHashCode(a333));
    }
}