用於 Java 中的類的修飾符

Question

我知道 Java 中的類僅使用“公共”和“默認（無修飾符）”，當您嘗試將 class 聲明為“私有”或“受保護”時，它會發出編譯錯誤，這對我來說很有意義。

但是，為什么這個頁面會說“類可以有上面提到的 4 個中的任何修飾符”。

有人可以向我解釋一下嗎？

Answer 1

還有更多的合法組合。 只是為了好玩，我寫了一個小腳本來生成所有可能的組合，然后我刪除了導致編譯錯誤的那些，這些是我留下的合法組合：

class Test {
                                             class     Default__ConcreteInstanceVirtualLoosefp_Class     {}
                                             interface Default__ConcreteInstanceVirtualLoosefp_Interface {}
                                             record    Default__ConcreteInstanceVirtualLoosefp_Record()  {}
                                    strictfp class     Default__ConcreteInstanceVirtualStrictfpClass     {}
                                    strictfp interface Default__ConcreteInstanceVirtualStrictfpInterface {}
                                    strictfp record    Default__ConcreteInstanceVirtualStrictfpRecord()  {}
                              final          class     Default__ConcreteInstanceFinal__Loosefp_Class     {}
                              final          record    Default__ConcreteInstanceFinal__Loosefp_Record()  {}
                              final strictfp class     Default__ConcreteInstanceFinal__StrictfpClass     {}
                              final strictfp record    Default__ConcreteInstanceFinal__StrictfpRecord()  {}
                       static                class     Default__ConcreteStatic__VirtualLoosefp_Class     {}
                       static                interface Default__ConcreteStatic__VirtualLoosefp_Interface {}
                       static                record    Default__ConcreteStatic__VirtualLoosefp_Record()  {}
                       static       strictfp class     Default__ConcreteStatic__VirtualStrictfpClass     {}
                       static       strictfp interface Default__ConcreteStatic__VirtualStrictfpInterface {}
                       static       strictfp record    Default__ConcreteStatic__VirtualStrictfpRecord()  {}
                       static final          class     Default__ConcreteStatic__Final__Loosefp_Class     {}
                       static final          record    Default__ConcreteStatic__Final__Loosefp_Record()  {}
                       static final strictfp class     Default__ConcreteStatic__Final__StrictfpClass     {}
                       static final strictfp record    Default__ConcreteStatic__Final__StrictfpRecord()  {}
              abstract                       class     Default__AbstractInstanceVirtualLoosefp_Class     {}
              abstract                       interface Default__AbstractInstanceVirtualLoosefp_Interface {}
              abstract              strictfp class     Default__AbstractInstanceVirtualStrictfpClass     {}
              abstract              strictfp interface Default__AbstractInstanceVirtualStrictfpInterface {}
              abstract static                class     Default__AbstractStatic__VirtualLoosefp_Class     {}
              abstract static                interface Default__AbstractStatic__VirtualLoosefp_Interface {}
              abstract static       strictfp class     Default__AbstractStatic__VirtualStrictfpClass     {}
              abstract static       strictfp interface Default__AbstractStatic__VirtualStrictfpInterface {}
    public                                   class     Public___ConcreteInstanceVirtualLoosefp_Class     {}
    public                                   interface Public___ConcreteInstanceVirtualLoosefp_Interface {}
    public                                   record    Public___ConcreteInstanceVirtualLoosefp_Record()  {}
    public                          strictfp class     Public___ConcreteInstanceVirtualStrictfpClass     {}
    public                          strictfp interface Public___ConcreteInstanceVirtualStrictfpInterface {}
    public                          strictfp record    Public___ConcreteInstanceVirtualStrictfpRecord()  {}
    public                    final          class     Public___ConcreteInstanceFinal__Loosefp_Class     {}
    public                    final          record    Public___ConcreteInstanceFinal__Loosefp_Record()  {}
    public                    final strictfp class     Public___ConcreteInstanceFinal__StrictfpClass     {}
    public                    final strictfp record    Public___ConcreteInstanceFinal__StrictfpRecord()  {}
    public             static                class     Public___ConcreteStatic__VirtualLoosefp_Class     {}
    public             static                interface Public___ConcreteStatic__VirtualLoosefp_Interface {}
    public             static                record    Public___ConcreteStatic__VirtualLoosefp_Record()  {}
    public             static       strictfp class     Public___ConcreteStatic__VirtualStrictfpClass     {}
    public             static       strictfp interface Public___ConcreteStatic__VirtualStrictfpInterface {}
    public             static       strictfp record    Public___ConcreteStatic__VirtualStrictfpRecord()  {}
    public             static final          class     Public___ConcreteStatic__Final__Loosefp_Class     {}
    public             static final          record    Public___ConcreteStatic__Final__Loosefp_Record()  {}
    public             static final strictfp class     Public___ConcreteStatic__Final__StrictfpClass     {}
    public             static final strictfp record    Public___ConcreteStatic__Final__StrictfpRecord()  {}
    public    abstract                       class     Public___AbstractInstanceVirtualLoosefp_Class     {}
    public    abstract                       interface Public___AbstractInstanceVirtualLoosefp_Interface {}
    public    abstract              strictfp class     Public___AbstractInstanceVirtualStrictfpClass     {}
    public    abstract              strictfp interface Public___AbstractInstanceVirtualStrictfpInterface {}
    public    abstract static                class     Public___AbstractStatic__VirtualLoosefp_Class     {}
    public    abstract static                interface Public___AbstractStatic__VirtualLoosefp_Interface {}
    public    abstract static       strictfp class     Public___AbstractStatic__VirtualStrictfpClass     {}
    public    abstract static       strictfp interface Public___AbstractStatic__VirtualStrictfpInterface {}
    private                                  class     Private__ConcreteInstanceVirtualLoosefp_Class     {}
    private                                  interface Private__ConcreteInstanceVirtualLoosefp_Interface {}
    private                                  record    Private__ConcreteInstanceVirtualLoosefp_Record()  {}
    private                         strictfp class     Private__ConcreteInstanceVirtualStrictfpClass     {}
    private                         strictfp interface Private__ConcreteInstanceVirtualStrictfpInterface {}
    private                         strictfp record    Private__ConcreteInstanceVirtualStrictfpRecord()  {}
    private                   final          class     Private__ConcreteInstanceFinal__Loosefp_Class     {}
    private                   final          record    Private__ConcreteInstanceFinal__Loosefp_Record()  {}
    private                   final strictfp class     Private__ConcreteInstanceFinal__StrictfpClass     {}
    private                   final strictfp record    Private__ConcreteInstanceFinal__StrictfpRecord()  {}
    private            static                class     Private__ConcreteStatic__VirtualLoosefp_Class     {}
    private            static                interface Private__ConcreteStatic__VirtualLoosefp_Interface {}
    private            static                record    Private__ConcreteStatic__VirtualLoosefp_Record()  {}
    private            static       strictfp class     Private__ConcreteStatic__VirtualStrictfpClass     {}
    private            static       strictfp interface Private__ConcreteStatic__VirtualStrictfpInterface {}
    private            static       strictfp record    Private__ConcreteStatic__VirtualStrictfpRecord()  {}
    private            static final          class     Private__ConcreteStatic__Final__Loosefp_Class     {}
    private            static final          record    Private__ConcreteStatic__Final__Loosefp_Record()  {}
    private            static final strictfp class     Private__ConcreteStatic__Final__StrictfpClass     {}
    private            static final strictfp record    Private__ConcreteStatic__Final__StrictfpRecord()  {}
    private   abstract                       class     Private__AbstractInstanceVirtualLoosefp_Class     {}
    private   abstract                       interface Private__AbstractInstanceVirtualLoosefp_Interface {}
    private   abstract              strictfp class     Private__AbstractInstanceVirtualStrictfpClass     {}
    private   abstract              strictfp interface Private__AbstractInstanceVirtualStrictfpInterface {}
    private   abstract static                class     Private__AbstractStatic__VirtualLoosefp_Class     {}
    private   abstract static                interface Private__AbstractStatic__VirtualLoosefp_Interface {}
    private   abstract static       strictfp class     Private__AbstractStatic__VirtualStrictfpClass     {}
    private   abstract static       strictfp interface Private__AbstractStatic__VirtualStrictfpInterface {}
    protected                                class     ProtectedConcreteInstanceVirtualLoosefp_Class     {}
    protected                                interface ProtectedConcreteInstanceVirtualLoosefp_Interface {}
    protected                                record    ProtectedConcreteInstanceVirtualLoosefp_Record()  {}
    protected                       strictfp class     ProtectedConcreteInstanceVirtualStrictfpClass     {}
    protected                       strictfp interface ProtectedConcreteInstanceVirtualStrictfpInterface {}
    protected                       strictfp record    ProtectedConcreteInstanceVirtualStrictfpRecord()  {}
    protected                 final          class     ProtectedConcreteInstanceFinal__Loosefp_Class     {}
    protected                 final          record    ProtectedConcreteInstanceFinal__Loosefp_Record()  {}
    protected                 final strictfp class     ProtectedConcreteInstanceFinal__StrictfpClass     {}
    protected                 final strictfp record    ProtectedConcreteInstanceFinal__StrictfpRecord()  {}
    protected          static                class     ProtectedConcreteStatic__VirtualLoosefp_Class     {}
    protected          static                interface ProtectedConcreteStatic__VirtualLoosefp_Interface {}
    protected          static                record    ProtectedConcreteStatic__VirtualLoosefp_Record()  {}
    protected          static       strictfp class     ProtectedConcreteStatic__VirtualStrictfpClass     {}
    protected          static       strictfp interface ProtectedConcreteStatic__VirtualStrictfpInterface {}
    protected          static       strictfp record    ProtectedConcreteStatic__VirtualStrictfpRecord()  {}
    protected          static final          class     ProtectedConcreteStatic__Final__Loosefp_Class     {}
    protected          static final          record    ProtectedConcreteStatic__Final__Loosefp_Record()  {}
    protected          static final strictfp class     ProtectedConcreteStatic__Final__StrictfpClass     {}
    protected          static final strictfp record    ProtectedConcreteStatic__Final__StrictfpRecord()  {}
    protected abstract                       class     ProtectedAbstractInstanceVirtualLoosefp_Class     {}
    protected abstract                       interface ProtectedAbstractInstanceVirtualLoosefp_Interface {}
    protected abstract              strictfp class     ProtectedAbstractInstanceVirtualStrictfpClass     {}
    protected abstract              strictfp interface ProtectedAbstractInstanceVirtualStrictfpInterface {}
    protected abstract static                class     ProtectedAbstractStatic__VirtualLoosefp_Class     {}
    protected abstract static                interface ProtectedAbstractStatic__VirtualLoosefp_Interface {}
    protected abstract static       strictfp class     ProtectedAbstractStatic__VirtualStrictfpClass     {}
    protected abstract static       strictfp interface ProtectedAbstractStatic__VirtualStrictfpInterface {}
}

因此，如果包括 14 個實驗record ，或單獨計算，則總共有 112 個合法組合， record s 的修飾符的合法組合為 32 個， interface s 也為 32 個，如果僅計算類，則為 48 個。

最明顯的非法組合是：

• abstract和final ： abstract的意思是“包含必須由子類實現的未實現的成員”，而final的意思是“不能被子類化”，很明顯，需要子類化但不能有子類的 class 沒有意義。
• 出於同樣的原因，您不能擁有final的interface interface隱式的完全abstract的，因為它應該由其實現class es 來實現。
• 相反， record是隱含的final ，因此它們不能是abstract 。

Answer 2

正如文中所說：

並非所有類都允許使用所有修飾符

但是，例如， 內部類可以是私有的/受保護的。

public class Foo {
    private Bar bar = new Bar(42);

    private static class Bar {
        private int answer;

        Bar(int theAnswer) {
            this.answer = theAnswer;
        }
    }
}

這對於一些持有數據的 POJO 或一些您想要封裝但不會泄漏的內部實現細節是有意義的（如果它受到保護，您甚至可以覆蓋）。