最近开始使用kotlin写项目了,这几天一直对object
,companion object
,内部类
和inner class
搞得有点迷糊
现在抽空,理清一下他们之间区别
我定义了一个class A
然后再class A内部分别定义了class B
,object C
,companion object D
,inner class E
四中不同的类
每个类中包含一个字段和一个函数
/**
* Copyright:AndroidInterview
* Author: liyang <br>
* Date:2019-11-12 14:29<br>
* Desc: <br>
*/
class A {
var sex = "男"
val age = 11
val a1:String="nan"
val a: String
get() {
return when (sex) {
"男" -> "man"
else -> "women"
}
}
fun a() {
//class B
//在A类中调用class B的b方法
val b = B()
b.b()
b.b
//inner class E
//在A类中调用inner class E的e变量和e()方法
val e=E()
e.e
e.e()
//object C
//在A类中调用object C的c()方法和c变量
C.c()
C.c
//companion object D
//可以直接在A类中调用伴生类的d()方法和d变量
d()
d
}
class B {
val b = "b"
fun b() {
println("b() is invoked")
}
}
object C {
const val c = "c"
fun c() {
println("c() is invoked")
}
}
companion object D {
val d = "d"
fun d() {
println("d() is invoked")
}
}
inner class E {
val e = "e"
fun e() {
println("e() is invoked")
}
}
}
接下来在main方法中调用它们,看看调用的时候有什么不同
/**
* Copyright:AndroidInterview
* Author: liyang <br>
* Date:2019-11-12 14:29<br>
* Desc: <br>
*/
open class Outer {
companion object {
@JvmStatic
fun main(args: Array<String>) {
val a = A()
a.a
a.a()
//内部类
val b = A.B()
println(b.b)
b.b()
//inner class
val e = a.E()
println(e.e)
e.e()
//object
println(A.C.c)
A.C.c()
//companion object
println(A.d)
A.d()
}
}
}
内部类类型 | 在A类内部调用 | 在A类外部调用 |
---|---|---|
object C |
C.c() ,C.c
|
A.C.c ,A.C.c()
|
companion object D | 可以直接使用 d ,d()
|
A.d ,A.d()
|
class B | 实例化B对象val b = B(); b.b() ; b.b
|
实例化B对象val b = A.B(); b.b() ; b.b
|
inner class E | 实例化E对象val e = E(); e.e() ; e.e
|
通过外部类的实例a来实例化E对象val e = a.E(); e.e() ; e.e
|
为什么这样?反编译看看源码就知道了
A.decompiled.java
import kotlin.Metadata;
import kotlin.jvm.internal.DefaultConstructorMarker;
import kotlin.jvm.internal.Intrinsics;
import org.jetbrains.annotations.NotNull;
@Metadata(
mv = {1, 1, 15},
bv = {1, 0, 3},
k = 1,
d1 = {"\u0000$\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0002\b\u0002\n\u0002\u0010\u000e\n\u0002\b\u0005\n\u0002\u0010\b\n\u0002\b\u0006\n\u0002\u0010\u0002\n\u0002\b\u0005\u0018\u0000 \u00142\u00020\u0001:\u0004\u0012\u0013\u0014\u0015B\u0005¢\u0006\u0002\u0010\u0002J\u0006\u0010\u0003\u001a\u00020\u0011R\u0011\u0010\u0003\u001a\u00020\u00048F¢\u0006\u0006\u001a\u0004\b\u0005\u0010\u0006R\u0014\u0010\u0007\u001a\u00020\u0004X\u0086D¢\u0006\b\n\u0000\u001a\u0004\b\b\u0010\u0006R\u0014\u0010\t\u001a\u00020\nX\u0086D¢\u0006\b\n\u0000\u001a\u0004\b\u000b\u0010\fR\u001a\u0010\r\u001a\u00020\u0004X\u0086\u000e¢\u0006\u000e\n\u0000\u001a\u0004\b\u000e\u0010\u0006\"\u0004\b\u000f\u0010\u0010¨\u0006\u0016"},
d2 = {"LA;", "", "()V", "a", "", "getA", "()Ljava/lang/String;", "a1", "getA1", "age", "", "getAge", "()I", "sex", "getSex", "setSex", "(Ljava/lang/String;)V", "", "B", "C", "D", "E", "baselib"}
)
public final class A {
@NotNull
private String sex = "男";
private final int age = 11;
@NotNull
private final String a1 = "nan";
@NotNull
private static final String d = "d";
public static final A.D D = new A.D((DefaultConstructorMarker)null);
@NotNull
public final String getSex() {
return this.sex;
}
public final void setSex(@NotNull String var1) {
Intrinsics.checkParameterIsNotNull(var1, "<set-?>");
this.sex = var1;
}
public final int getAge() {
return this.age;
}
@NotNull
public final String getA1() {
return this.a1;
}
@NotNull
public final String getA() {
String var1 = this.sex;
String var10000;
switch(var1.hashCode()) {
case 30007:
if (var1.equals("男")) {
var10000 = "man";
break;
}
default:
var10000 = "women";
}
return var10000;
}
public final void a() {
A.B b = new A.B();
b.b();
b.getB();
A.C.INSTANCE.c();
D.d();
String var10000 = d;
}
@Metadata(
mv = {1, 1, 15},
bv = {1, 0, 3},
k = 1,
d1 = {"\u0000\u001a\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0002\b\u0002\n\u0002\u0010\u000e\n\u0002\b\u0002\n\u0002\u0010\u0002\n\u0000\u0018\u00002\u00020\u0001B\u0005¢\u0006\u0002\u0010\u0002J\u0006\u0010\u0003\u001a\u00020\u0007R\u0014\u0010\u0003\u001a\u00020\u0004X\u0086D¢\u0006\b\n\u0000\u001a\u0004\b\u0005\u0010\u0006¨\u0006\b"},
d2 = {"LA$B;", "", "()V", "b", "", "getB", "()Ljava/lang/String;", "", "baselib"}
)
public static final class B {
@NotNull
private final String b = "b";
@NotNull
public final String getB() {
return this.b;
}
public final void b() {
String var1 = "b() is invoked";
boolean var2 = false;
System.out.println(var1);
}
}
@Metadata(
mv = {1, 1, 15},
bv = {1, 0, 3},
k = 1,
d1 = {"\u0000\u0016\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0002\b\u0002\n\u0002\u0010\u000e\n\u0002\u0010\u0002\n\u0000\bÆ\u0002\u0018\u00002\u00020\u0001B\u0007\b\u0002¢\u0006\u0002\u0010\u0002J\u0006\u0010\u0003\u001a\u00020\u0005R\u000e\u0010\u0003\u001a\u00020\u0004X\u0086T¢\u0006\u0002\n\u0000¨\u0006\u0006"},
d2 = {"LA$C;", "", "()V", "c", "", "", "baselib"}
)
public static final class C {
@NotNull
public static final String c = "c";
public static final A.C INSTANCE;
public final void c() {
String var1 = "c() is invoked";
boolean var2 = false;
System.out.println(var1);
}
private C() {
}
static {
A.C var0 = new A.C();
INSTANCE = var0;
}
}
@Metadata(
mv = {1, 1, 15},
bv = {1, 0, 3},
k = 1,
d1 = {"\u0000\u001a\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0002\b\u0002\n\u0002\u0010\u000e\n\u0002\b\u0002\n\u0002\u0010\u0002\n\u0000\b\u0086\u0004\u0018\u00002\u00020\u0001B\u0005¢\u0006\u0002\u0010\u0002J\u0006\u0010\u0003\u001a\u00020\u0007R\u0014\u0010\u0003\u001a\u00020\u0004X\u0086D¢\u0006\b\n\u0000\u001a\u0004\b\u0005\u0010\u0006¨\u0006\b"},
d2 = {"LA$E;", "", "(LA;)V", "e", "", "getE", "()Ljava/lang/String;", "", "baselib"}
)
public final class E {
@NotNull
private final String e = "e";
@NotNull
public final String getE() {
return this.e;
}
public final void e() {
String var1 = "e() is invoked";
boolean var2 = false;
System.out.println(var1);
}
}
@Metadata(
mv = {1, 1, 15},
bv = {1, 0, 3},
k = 1,
d1 = {"\u0000\u001a\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0002\b\u0002\n\u0002\u0010\u000e\n\u0002\b\u0002\n\u0002\u0010\u0002\n\u0000\b\u0086\u0003\u0018\u00002\u00020\u0001B\u0007\b\u0002¢\u0006\u0002\u0010\u0002J\u0006\u0010\u0003\u001a\u00020\u0007R\u0014\u0010\u0003\u001a\u00020\u0004X\u0086D¢\u0006\b\n\u0000\u001a\u0004\b\u0005\u0010\u0006¨\u0006\b"},
d2 = {"LA$D;", "", "()V", "d", "", "getD", "()Ljava/lang/String;", "", "baselib"}
)
public static final class D {
@NotNull
public final String getD() {
return A.d;
}
public final void d() {
String var1 = "d() is invoked";
boolean var2 = false;
System.out.println(var1);
}
private D() {
}
// $FF: synthetic method
public D(DefaultConstructorMarker $constructor_marker) {
this();
}
}
}
通过查看decompile A.kt文件之后得到的A.decompiled.java源文件
可以得到以下几个重要信息
- kotlin中只要不是被inner修饰的内部类,都是静态的!所以
class B
,object C
,companion object D
都是静态内部类 -
inner class E
是非静态内部类,所以它会持有A的引用 -
object C
和companion object D
的构造函数都是private
的 - 无论是
object
还是companion object
中的变量都是静态常量
梳理下
内部类类型 | 是否可以定义多个 | 是否是单例 | 是否是静态内部类 |
---|---|---|---|
class | 是 | 否 | 是 |
inner class | 是 | 否 | 否 |
object | 是 | 是 | 是 |
companion object | 否 | 是 | 是 |
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