Java中的默认值和初始化

Question

根据我的参考,基本类型具有默认值,对象为空.我测试了一段代码.

public class Main {
    public static void main(String[] args) {
        int a;
        System.out.println(a);
    }
}

该行将System.out.println(a);是指向变量的错误a,variable a might not have been initialized而在给定的引用中,该变量integer将具有0默认值.但是,使用下面给出的代码,它实际上会打印出来0.

public class Main {
    static int a;
    public static void main(String[] args) {
        System.out.println(a);
    }
}

第一个代码可能出现什么问题？类实例变量的行为是否与局部变量不同？

Answer 1

在第一个代码示例中,a是一个main方法局部变量.方法局部变量在使用之前需要初始化.

在第二个代码示例中,a是类成员变量,因此它将初始化为默认值.

Answer 2

仔细阅读您的参考:

默认值

声明字段时并不总是需要分配值. 声明但未初始化的字段将由编译器设置为合理的默认值.一般来说,此默认值将为零或null,具体取决于数据类型.然而,依赖于这样的默认值通常被认为是糟糕的编程风格.

下表总结了上述数据类型的默认值.

...

局部变量略有不同; 编译器永远不会为未初始化的局部变量分配默认值.如果无法初始化声明它的局部变量,请确保在尝试使用它之前为其赋值.访问未初始化的局部变量将导致编译时错误.

Answer 3

这些是涉及的主要因素:

1. 成员变量(默认OK)
2. 静态变量(默认OK)
3. 最终成员变量(未初始化,必须在构造函数上设置)
4. 最终的静态变量(未初始化,必须在静态块{}上设置)
5. 局部变量(未初始化)

注1:您必须在每个实现的构造函数上初始化最终成员变量!

注意2:您必须在构造函数本身的块内初始化最终成员变量,而不是调用另一个初始化它们的方法.例如,这是无效的:

private final int memberVar;

public Foo() {
    //invalid initialization of a final member
    init();
}

private void init() {
    memberVar = 10;
}

注3:数组是Java中的对象,即使它们存储基元.

注意4:初始化数组时,其所有项都设置为默认值,与成员或本地数组无关.

我附上一个代码示例,介绍上述情况:

public class Foo {
    //static and member variables are initialized to default values

    //primitives
    private int a; //default 0
    private static int b; //default 0

    //objects
    private Object c; //default NULL
    private static Object d; //default NULL

    //arrays (Note: they are objects too, even if they store primitives)
    private int[] e; //default NULL
    private static int[] f; //default NULL

    //what if declared as final?

    //primitives
    private final int g; //not initialized, MUST set in constructor
    private final static int h; //not initialized, MUST set in a static {}

    //objects
    private final Object i; //not initialized, MUST set in constructor
    private final static Object j; //not initialized, MUST set in a static {}

    //arrays
    private final int[] k; //not initialized, MUST set in constructor
    private final static int[] l; //not initialized, MUST set in a static {}

    //initialize final statics
    static {
        h = 5;
        j = new Object();
        l = new int[5]; //elements of l are initialized to 0
    }

    //initialize final member variables
    public Foo() {
        g = 10;
        i = new Object();
        k = new int[10]; //elements of k are initialized to 0
    }

    //A second example constructor
    //you have to initialize final member variables to every constructor!
    public Foo(boolean aBoolean) {
        g = 15;
        i = new Object();
        k = new int[15]; //elements of k are initialized to 0
    }

    public static void main(String[] args) {
        //local variables are not initialized
        int m; //not initialized
        Object n; //not initialized
        int[] o; //not initialized

        //we must initialize them before usage
        m = 20;
        n = new Object();
        o = new int[20]; //elements of o are initialized to 0
    }
}

Answer 4

在声明原始类型值时,需要记住一些事项.

他们是:

1. 在方法内声明的值不会被赋予默认值.
2. 声明为实例变量或静态变量的值将分配默认值0.

所以在你的代码中:

public class Main {
    int instanceVariable;
    static int staticVariable;
    public static void main(String[] args) {
        Main mainInstance = new Main() 
        int localVariable;
        int localVariableTwo = 2;
        System.out.println(mainInstance.instanceVariable);
        System.out.println(staticVariable);
       // System.out.println(localVariable); //will throw compilation error
        System.out.println(localVariableTwo);

    }
}