Java 修饰符

访问控制修饰符

Four modifiers decide visibility. default is not a keyword but the state when you write nothing — visible within the same package only.

Modifier	Class	Package	Subclass	World
private	✔	✘	✘	✘
default	✔	✔	✘	✘
protected	✔	✔	✔	✘
public	✔	✔	✔	✔

Java

public class Access {
    private int a = 1;
    int b = 2;
    protected int c = 3;
    public int d = 4;

    public static void main(String[] args) {
        Access o = new Access();
        System.out.println(o.a + o.b + o.c + o.d);
    }
}

Output

非访问修饰符

static makes a member belong to the class, final freezes a variable, method or class, abstract declares an unimplemented method, synchronized locks a method for thread safety, and transient skips a field during serialization.

Modifier	Applies to	Effect
static	field / method / block	Belongs to the class
final	field / method / class	No reassign / override / extend
abstract	class / method	Must be implemented by subclass
synchronized	method / block	Mutual exclusion lock
transient	field	Excluded from serialization
volatile	field	Always read from main memory

Java

public class NonAccess {
    static int counter = 0;
    final int limit = 5;

    static void inc() {
        counter++;
    }

    public static void main(String[] args) {
        inc();
        inc();

        System.out.println(counter);
        System.out.println(new NonAccess().limit);
    }
}

Output

2
5

final on a reference forbids reassignment, not mutation: final List<String> list still allows list.add(...).

访问控制修饰符

四个修饰符决定成员能被谁访问。default 不是关键字，而是「什么都不写」时的默认状态，只有同包可见。

修饰符	同类	同包	子类	其他
private	✔	✘	✘	✘
default	✔	✔	✘	✘
protected	✔	✔	✔	✘
public	✔	✔	✔	✔

Java

public class Access {
    private int a = 1;
    int b = 2;
    protected int c = 3;
    public int d = 4;

    public static void main(String[] args) {
        Access o = new Access();
        System.out.println(o.a + o.b + o.c + o.d);
    }
}

Output

非访问修饰符

static 让成员属于类；final 冻结变量、方法或类；abstract 声明未实现的抽象方法；synchronized 给方法加锁保证线程安全；transient 跳过序列化。

修饰符	作用对象	效果
static	字段 / 方法 / 代码块	属于类
final	字段 / 方法 / 类	不可改 / 重写 / 继承
abstract	类 / 方法	子类必须实现
synchronized	方法 / 代码块	互斥锁
transient	字段	不参与序列化
volatile	字段	总从主内存读取

Java

public class NonAccess {
    static int counter = 0;
    final int limit = 5;

    static void inc() {
        counter++;
    }

    public static void main(String[] args) {
        inc();
        inc();

        System.out.println(counter);
        System.out.println(new NonAccess().limit);
    }
}

Output

2
5

final 修饰引用时，引用不能改指向，但对象内部仍可变 —— final List<String> list 依然可以 list.add(...)。

Java 修饰符

访问控制修饰符

非访问修饰符

访问控制修饰符

非访问修饰符

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