在 Java 中用于 类 的修饰符
Modifiers used for Classes in Java
我知道 class 中的 Java 仅使用“public”和“默认（无修饰符）”，当您尝试声明 class作为“私有”或“受保护”，它给出了一个编译错误，这对我来说很有意义。
但是，为什么 this page 说“classes 可以有上述 4 个修饰符中的任何修饰符”。
谁能给我解释一下吗？
如文中所说：
Not all modifiers are allowed on all classes
但是，例如，inner classes 可以是 private/protected。
public class Foo {
private Bar bar = new Bar(42);
private static class Bar {
private int answer;
Bar(int theAnswer) {
this.answer = theAnswer;
}
}
}
这对于某些保存数据的 POJO 或您想要封装但不会泄漏的某些内部实现细节可能有意义（如果它受到保护，您甚至可以覆盖）。
还有更多的合法组合。只是为了好玩，我写了一个生成所有可能组合的小脚本，然后删除了导致编译错误的那些，这些是我剩下的合法组合：
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 {}
}
所以，如果算上Java14个实验性的record，或者单独计算的话，总共有112个合法的组合，32个record修饰符的合法组合，32以及 interfaces，如果你只计算 classes.
则为 48
最明显的非法组合是：
abstract和final：abstract表示“包含un-implemented个必须由subclasses实现的成员”和final意思是“不能被 subclassed”，很明显，需要 subclassing 但不能有 subclasses 的 class 没有意义。- 出于同样的原因，您不能拥有
final 的 interface：interface 隐式完全 abstract，因为它应该是通过其实施 classes. 实施
- 并且相反，
records 是隐含的 final，所以它们不能是 abstract。
我知道 class 中的 Java 仅使用“public”和“默认（无修饰符）”，当您尝试声明 class作为“私有”或“受保护”，它给出了一个编译错误，这对我来说很有意义。
但是，为什么 this page 说“classes 可以有上述 4 个修饰符中的任何修饰符”。
谁能给我解释一下吗？
如文中所说：
Not all modifiers are allowed on all classes
但是，例如，inner classes 可以是 private/protected。
public class Foo {
private Bar bar = new Bar(42);
private static class Bar {
private int answer;
Bar(int theAnswer) {
this.answer = theAnswer;
}
}
}
这对于某些保存数据的 POJO 或您想要封装但不会泄漏的某些内部实现细节可能有意义（如果它受到保护，您甚至可以覆盖）。
还有更多的合法组合。只是为了好玩，我写了一个生成所有可能组合的小脚本，然后删除了导致编译错误的那些，这些是我剩下的合法组合：
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 {}
}
所以，如果算上Java14个实验性的record，或者单独计算的话，总共有112个合法的组合，32个record修饰符的合法组合，32以及 interfaces，如果你只计算 classes.
最明显的非法组合是：
abstract和final：abstract表示“包含un-implemented个必须由subclasses实现的成员”和final意思是“不能被 subclassed”，很明显，需要 subclassing 但不能有 subclasses 的 class 没有意义。- 出于同样的原因，您不能拥有
final的interface：interface隐式完全abstract，因为它应该是通过其实施classes. 实施
- 并且相反，
records 是隐含的final，所以它们不能是abstract。