我不明白为什么在第一个代码（代码1）中，mycounter指向的对象在传递到方法“print”后被更新为值2。但是在第二个代码（代码2）中，str指向的对象仍然是相同的文本“这是一个字符串文本”。我认为str（str是一个对象引用，我认为与mycounter类似）经历了相同的机制，因为它也被传递给一个方法，所以它不应该像mycounter一样被更新吗？
这是代码1：

public class PrimitiveVsReference{

private static class Counter {
    private int count;

    public void advance(int number) {
        count += number;
    }

    public int getCount() {
        return count;
    }
}

public static void main(String args[]) {        
    int i = 30;
    System.out.println("value of i before passing to method : " + i);
    print(30);
    System.out.println("value of i after passing to method : " + i);

    Counter myCounter = new Counter();
    System.out.println("counter before passing to method : " + myCounter.getCount());// this gives 0
    print(myCounter); 
    System.out.println("counter after passing to method : " + myCounter.getCount());// now this gives 2 after passing into the method "print"
}

/*
 * print given reference variable's value
 */
public static void print(Counter ctr) {
    ctr.advance(2);
}

/**
 * print given primitive value
 */
public static void print(int value) {
    value++;
}
}

代码2：

String str = "This is a string literal.";

public static void tryString(String s)
{
    s = "a different string";
}

tryString(str); // isn't this here doing the samething as when myCounter is passed to print in Code 1?
System.out.println("str = " + str); // But this here output the original literal "This is a string literal."

有人能解释一下发生了什么事吗？