java - 如何在 Java 中使用指针？

Question

我知道 Java 没有指针，但我听说 Java 程序可以使用指针创建，并且这可以由少数 Java 专家来完成。是真的吗？

Answer 1

Java 中的所有对象都是引用，您可以像使用指针一样使用它们。

abstract class Animal
{...
}

class Lion extends Animal
{...
}

class Tiger extends Animal
{   
public Tiger() {...}
public void growl(){...}
}

Tiger first = null;
Tiger second = new Tiger();
Tiger third;

解除对空值的引用:

first.growl();  // ERROR, first is null.    
third.growl(); // ERROR, third has not been initialized.

别名问题:

third = new Tiger();
first = third;

丢失的细胞:

second = third; // Possible ERROR. The old value of second is lost.    

您可以通过首先确保不再需要 second 的旧值或为另一个指针分配 second 的值来确保安全。

first = second;
second = third; //OK

请注意，以其他方式(NULL、new...)给第二个值同样是一个潜在的错误，并可能导致丢失它指向的对象。

当你调用 new 并且分配器无法分配请求的单元格时，Java 系统会抛出异常(OutOfMemoryError)。这是非常罕见的，通常是由于递归失控造成的。

请注意，从语言的角度来看，将对象丢弃到垃圾收集器根本不是错误。这只是程序员需要注意的事情。同一个变量可以在不同的时间指向不同的对象，当没有指针引用它们时，旧值将被回收。但是如果程序的逻辑需要维护至少一个对象的引用，就会报错。

新手常犯如下错误。

Tiger tony = new Tiger();
tony = third; // Error, the new object allocated above is reclaimed. 

你可能想说的是:

Tiger tony = null;
tony = third; // OK.

不正确的类型转换:

Lion leo = new Lion();
Tiger tony = (Tiger)leo; // Always illegal and caught by compiler. 

Animal whatever = new Lion(); // Legal.
Tiger tony = (Tiger)whatever; // Illegal, just as in previous example.
Lion leo = (Lion)whatever; // Legal, object whatever really is a Lion.

---

C 中的指针:

void main() {   
    int*    x;  // Allocate the pointers x and y
    int*    y;  // (but not the pointees)

    x = malloc(sizeof(int));    // Allocate an int pointee,
                                // and set x to point to it

    *x = 42;    // Dereference x to store 42 in its pointee

    *y = 13;    // CRASH -- y does not have a pointee yet

    y = x;      // Pointer assignment sets y to point to x's pointee

    *y = 13;    // Dereference y to store 13 in its (shared) pointee
}

Java 中的指针:

class IntObj {
    public int value;
}

public class Binky() {
    public static void main(String[] args) {
        IntObj  x;  // Allocate the pointers x and y
        IntObj  y;  // (but not the IntObj pointees)

        x = new IntObj();   // Allocate an IntObj pointee
                            // and set x to point to it

        x.value = 42;   // Dereference x to store 42 in its pointee

        y.value = 13;   // CRASH -- y does not have a pointee yet

        y = x;  // Pointer assignment sets y to point to x's pointee

        y.value = 13;   // Deference y to store 13 in its (shared) pointee
    }
} 

更新: 正如评论中所建议的，必须注意 C 具有指针算法。但是，我们在 Java 中没有。