内存笔试题

知识点：方法调用时值传递
        栈内存中，方法调用完，参数立即消失！
        堆内存不一定立即消失，由GC垃圾回收！

第一题;

public class ChangeStr {

	public static void changStr(String str){
		str = "welcome";
	}
	public static void main(String[] args) {
		String str = "1234";
		changStr(str);
		System.out.println(str);
	}
}

========================

输出结果：1234

分析提示：局部变量存在栈内存中，实例变量分配在堆内存！

======================================
第二题：

package test1;

public class Change {
	String str = new String("good");
	char[] ch = {'a','b','c'};
	
	
	public void change(String str,char ch[]){
		str = "test ok";
//		char[] ch2 = {'s','d','f'};
		ch[0] = 'g';
//		ch = ch2;
	}

	
	public static void main(String[] args) {	
		
		Change ex = new Change();
		ex.change(ex.str, ex.ch);
		
		System.out.println(ex.str); // here
		System.out.println(ex.ch); // here
		
	
	}
}

运行结果：
          good
          gbc

======================================
第二题在方法体内重新创建一个数组，进行赋值

package test1;

public class Change {
	String str = new String("good");
	char[] ch = { 'a', 'b', 'c' };

	public void change(String str, char ch[]) {
		str = "test ok";
               //改变了一下，
		char[] ch2 = { 's', 'd', 'f' };
		// ch[0] = 'g';
		ch = ch2;
	}

		public static void main(String[] args) {

		Change ex = new Change();
		ex.change(ex.str, ex.ch);

		System.out.println("1 ex.str = " + ex.str);
		System.out.println("2 ex.ch = " + ex.ch.toString());
		System.out.println("3 ex.ch = " + ex.ch);---> warming: Must explicitly convert the char[] to a String
		System.out.println("4 ex.ch = " + String.valueOf(ex.ch));
		System.out.println( ex.ch);
		System.out.println( ex.ch.toString());
		for (char c : ex.ch) {
			System.out.println("for each ex.ch = " + c);
		}
	}
}

运行结果：1 ex.str = good
          2 ex.ch = [C@de6ced
          3 ex.ch = [C@de6ced
          4 ex.ch = abc
          abc
          [C@de6ced
          for each ex.ch = a
          for each ex.ch = b
          for each ex.ch = c

======================================

可用throws抛出的异常是：BC
A.Error     --->系统及异常
B.Exception --->已检查异常 必须捕获和处理
C.Throwable
D.RuntimeException --->未检查异常，运行时异常，不需要马上捕获


java.lang
Class Throwable
  |
   ---java.lang.Object
  java.lang.Throwable

All Implemented Interfaces:
Serializable
Direct Known Subclasses:
Error, Exception

Throwable：Instances of two subclasses, java.lang.Error and java.lang.Exception

java.lang
Class Error
java.lang.Object
  java.lang.Throwable
      java.lang.Error



java.lang
Class RuntimeException
java.lang.Object
  java.lang.Throwable
      java.lang.Exception
          java.lang.RuntimeException



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