kotlin data class 和 class 区别

kotlin
kotlin

kotlin data class 和 java class 区别

使用限制

data class 必须要有带参数的构造方法 （Data class must have at least one primary constructor parameter）

data class 不能被继承（Modifier ‘data’ is incompatible with ‘open’）

实现区别

普通class

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class Nearby(val age: Int)

//转成java

public final class Nearby {
   private final int age;

   public final int getAge() {
      return this.age;
   }

   public Nearby(int age) {
      this.age = age;
   }
}

data class

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data class Nearby(val age: Int)

//转成java

public final class Nearby {
   private final int age;

   public final int getAge() {
      return this.age;
   }

   public Nearby(int age) {
      this.age = age;
   }

   public final int component1() {
      return this.age;
   }

   @NotNull
   public final Nearby copy(int age) {
      return new Nearby(age);
   }

   // $FF: synthetic method
   public static Nearby copy$default(Nearby var0, int var1, int var2, Object var3) {
      if ((var2 & 1) != 0) {
         var1 = var0.age;
      }

      return var0.copy(var1);
   }

   @NotNull
   public String toString() {
      return "Nearby(age=" + this.age + ")";
   }

   public int hashCode() {
      return Integer.hashCode(this.age);
   }

   public boolean equals(@Nullable Object var1) {
      if (this != var1) {
         if (var1 instanceof Nearby) {
            Nearby var2 = (Nearby)var1;
            if (this.age == var2.age) {
               return true;
            }
         }

         return false;
      } else {
         return true;
      }
   }
}

两相对比，data class比class 多实现了 toString()、hashCode()、equals()、copy()、componentN()方法。
hashCode()、equals()是用来比较对象内容是否相同，多用于HashMap等容器中；
toString()是用来打印对象内容；
copy()实现了复制功能；
componentN()提供了快速访问元素的功能。
从上面看data class的功能 class都能实现，data class只是是kotlin提供的具有常用数据model功能的类，用于提升开发效率。

一点拓展

data class 可以有普通变量吗？答案是可以，那普通变量和元素变量有何区别呢？看下面的例子：

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data class Nearby(val age: Int) {
    var name = "nearby"
}

///转成java

public final class Nearby {
   @NotNull
   private String name;
   private final int age;

   @NotNull
   public final String getName() {
      return this.name;
   }

   public final void setName(@NotNull String var1) {
      Intrinsics.checkNotNullParameter(var1, "<set-?>");
      this.name = var1;
   }

   public final int getAge() {
      return this.age;
   }

   public Nearby(int age) {
      this.age = age;
      this.name = "nearby";
   }

   public final int component1() {
      return this.age;
   }

   @NotNull
   public final Nearby copy(int age) {
      return new Nearby(age);
   }

   // $FF: synthetic method
   public static Nearby copy$default(Nearby var0, int var1, int var2, Object var3) {
      if ((var2 & 1) != 0) {
         var1 = var0.age;
      }

      return var0.copy(var1);
   }

   @NotNull
   public String toString() {
      return "Nearby(age=" + this.age + ")";
   }

   public int hashCode() {
      return Integer.hashCode(this.age);
   }

   public boolean equals(@Nullable Object var1) {
      if (this != var1) {
         if (var1 instanceof Nearby) {
            Nearby var2 = (Nearby)var1;
            if (this.age == var2.age) {
               return true;
            }
         }

         return false;
      } else {
         return true;
      }
   }
}