枚举和注解
枚举类
把对象一个一个列出来的类称为枚举类，枚举对应英文enum，是一组常量的集合，属于一种特殊的类，里面只包含一组有限的特定的对象(可以作为switch返回值)
自定义实现方法及特点：
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1、将构造器私有化
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2、删去setXxx方法，可以有get方法
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3、对枚举对象/属性用final + static 修饰，实现优化(在使用枚举对象的时候不需要加载枚举类)
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4、枚举对象根据需要，可以创建多个属性
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枚举名一般都大写(final修饰的都大写)
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实例：
public class Enum01 { public static void main(String[] args) { System.out.println(Season.AUTUMN.getName()); System.out.println(Season.SPRING.getName()); } } //演示枚举实现 class Season{ private String name; private String desc; //1、将构造器私有化，防止直接new private Season(String name, String desc) { this.name = name; this.desc = desc; } //2、去掉setXxx方法，防止属性修改 // // public void setName(String name) { // this.name = name; // } // // public void setDesc(String desc) { // this.desc = desc; // } public String getName() { return name; } public String getDesc() { return desc; } //3、在Season内部直接创建固定的对象 //4、若使用静态属性会导致Season加载，若加上final，不会导致类加载 public static final Season SPRING = new Season("Spring", "Warm"); public static final Season SUMMER = new Season("Summer", "Hot"); public static final Season AUTUMN = new Season("Autumn", "不舒服"); public static final Season WINTER = new Season("Winter", "Warm"); }
用关键字enum来实现枚举类
使用方法：
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1、使用关键字enum来代替class
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2、直接使用对象名(属性…)来创建对象相当于替代了public static final Season SPRING = new Season(“Spring”, “Warm”);
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3、若有多个对象，用,隔开
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4、若用enum实现枚举，要求将定义常量对象写在最前面
举例：
public class Enum02 {
public static void main(String[] args) {
System.out.println(Season.SPRING);
System.out.println(Season.SUMMER);
System.out.println(Season.AUTUMN);
System.out.println(Season.WINTER);
}
}
enum Season{
SPRING("春天", "温暖"),SUMMER("夏天","热"),
AUTUMN("秋天","凉爽"),WINTER("冬天","冷");
private String name;//名字
private String desc;//描述
Season(String name, String desc) {
this.name = name;
this.desc = desc;
}
@Override
public String toString() {
return "Season{" +
"name='" + name + '\'' +
", desc='" + desc + '\'' +
'}';
}
}
注意事项：
练习：
public class Exercise {
public static void main(String[] args) {
Gender boy = Gender.BOY;
Gender boy2 = Gender.BOY;
System.out.println(boy);
//Gender中没有toString方法，因此会调用父类中的toString()方法，即Enum中的toString
//可查看toString源码得知toString()方法返回的是name，即类名
System.out.println(boy == boy2);
}
}
//输出：
BOY
true
enum Gender{
BOY, GIRL;
//无属性，无构造器使用默认构造器，因此这种写法是正确的
}
练习二：(枚举与switch搭配使用)
public class Homework08 {
public static void main(String[] args) {
//枚举值与switch搭配使用
Color green = Color.GREEN;
green.show();
switch (green){
case YELLOW:
System.out.println("匹配到...");
break;
default:
System.out.println("无");
}
}
}
enum Color implements Beauty{
RED(255,0,0),
BLUE(0,0,255),
BLACK(0,0,0),
YELLOW(255,255,0),
GREEN(0,255,0);
private int redValue;
private int greenValue;
private int blueValue;
private Color(int redValue, int greenValue, int blueValue) {
this.redValue = redValue;
this.greenValue = greenValue;
this.blueValue = blueValue;
}
@Override
public void show() {
System.out.println("redValue:" + redValue
+ "\tgreenValue:" + greenValue
+ "\tblueValue:" + blueValue);
}
}
interface Beauty{
void show();
}
enum常用方法说明，因为继承了Enum类，我们可以使用Enum类相关的方法，Enum源码如下：
public abstract class Enum<E extends Enum<E>>
implements Comparable<E>, Serializable {
/**
* The name of this enum constant, as declared in the enum declaration.
* Most programmers should use the {@link #toString} method rather than
* accessing this field.
*/
private final String name;
/**
* Returns the name of this enum constant, exactly as declared in its
* enum declaration.
*
* <b>Most programmers should use the {@link #toString} method in
* preference to this one, as the toString method may return
* a more user-friendly name.</b> This method is designed primarily for
* use in specialized situations where correctness depends on getting the
* exact name, which will not vary from release to release.
*
* @return the name of this enum constant
*/
public final String name() {
return name;
}
/**
* The ordinal of this enumeration constant (its position
* in the enum declaration, where the initial constant is assigned
* an ordinal of zero).
*
* Most programmers will have no use for this field. It is designed
* for use by sophisticated enum-based data structures, such as
* {@link java.util.EnumSet} and {@link java.util.EnumMap}.
*/
private final int ordinal;
/**
* Returns the ordinal of this enumeration constant (its position
* in its enum declaration, where the initial constant is assigned
* an ordinal of zero).
*
* Most programmers will have no use for this method. It is
* designed for use by sophisticated enum-based data structures, such
* as {@link java.util.EnumSet} and {@link java.util.EnumMap}.
*
* @return the ordinal of this enumeration constant
*/
public final int ordinal() {
return ordinal;
}
/**
* Sole constructor. Programmers cannot invoke this constructor.
* It is for use by code emitted by the compiler in response to
* enum type declarations.
*
* @param name - The name of this enum constant, which is the identifier
* used to declare it.
* @param ordinal - The ordinal of this enumeration constant (its position
* in the enum declaration, where the initial constant is assigned
* an ordinal of zero).
*/
protected Enum(String name, int ordinal) {
this.name = name;
this.ordinal = ordinal;
}
/**
* Returns the name of this enum constant, as contained in the
* declaration. This method may be overridden, though it typically
* isn't necessary or desirable. An enum type should override this
* method when a more "programmer-friendly" string form exists.
*
* @return the name of this enum constant
*/
public String toString() {
return name;
}
/**
* Returns true if the specified object is equal to this
* enum constant.
*
* @param other the object to be compared for equality with this object.
* @return true if the specified object is equal to this
* enum constant.
*/
public final boolean equals(Object other) {
return this==other;
}
/**
* Returns a hash code for this enum constant.
*
* @return a hash code for this enum constant.
*/
public final int hashCode() {
return super.hashCode();
}
/**
* Throws CloneNotSupportedException. This guarantees that enums
* are never cloned, which is necessary to preserve their "singleton"
* status.
*
* @return (never returns)
*/
protected final Object clone() throws CloneNotSupportedException {
throw new CloneNotSupportedException();
}
/**
* Compares this enum with the specified object for order. Returns a
* negative integer, zero, or a positive integer as this object is less
* than, equal to, or greater than the specified object.
*
* Enum constants are only comparable to other enum constants of the
* same enum type. The natural order implemented by this
* method is the order in which the constants are declared.
*/
public final int compareTo(E o) {
Enum<?> other = (Enum<?>)o;
Enum<E> self = this;
if (self.getClass() != other.getClass() && // optimization
self.getDeclaringClass() != other.getDeclaringClass())
throw new ClassCastException();
return self.ordinal - other.ordinal;
}
/**
* Returns the Class object corresponding to this enum constant's
* enum type. Two enum constants e1 and e2 are of the
* same enum type if and only if
* e1.getDeclaringClass() == e2.getDeclaringClass().
* (The value returned by this method may differ from the one returned
* by the {@link Object#getClass} method for enum constants with
* constant-specific class bodies.)
*
* @return the Class object corresponding to this enum constant's
* enum type
*/
@SuppressWarnings("unchecked")
public final Class<E> getDeclaringClass() {
Class<?> clazz = getClass();
Class<?> zuper = clazz.getSuperclass();
return (zuper == Enum.class) ? (Class<E>)clazz : (Class<E>)zuper;
}
/**
* Returns the enum constant of the specified enum type with the
* specified name. The name must match exactly an identifier used
* to declare an enum constant in this type. (Extraneous whitespace
* characters are not permitted.)
*
* <p>Note that for a particular enum type {@code T}, the
* implicitly declared {@code public static T valueOf(String)}
* method on that enum may be used instead of this method to map
* from a name to the corresponding enum constant. All the
* constants of an enum type can be obtained by calling the
* implicit {@code public static T[] values()} method of that
* type.
*
* @param <T> The enum type whose constant is to be returned
* @param enumType the {@code Class} object of the enum type from which
* to return a constant
* @param name the name of the constant to return
* @return the enum constant of the specified enum type with the
* specified name
* @throws IllegalArgumentException if the specified enum type has
* no constant with the specified name, or the specified
* class object does not represent an enum type
* @throws NullPointerException if {@code enumType} or {@code name}
* is null
* @since 1.5
*/
public static <T extends Enum<T>> T valueOf(Class<T> enumType,
String name) {
T result = enumType.enumConstantDirectory().get(name);
if (result != null)
return result;
if (name == null)
throw new NullPointerException("Name is null");
throw new IllegalArgumentException(
"No enum constant " + enumType.getCanonicalName() + "." + name);
}
/**
* enum classes cannot have finalize methods.
*/
protected final void finalize() { }
/**
* prevent default deserialization
*/
private void readObject(ObjectInputStream in) throws IOException,
ClassNotFoundException {
throw new InvalidObjectException("can't deserialize enum");
}
private void readObjectNoData() throws ObjectStreamException {
throw new InvalidObjectException("can't deserialize enum");
}
}
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name方法(返回此枚举常量的名称)
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ordinal方法(返回此枚举常量的序号)
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values方法 Season.values()，返回一个此类中所有常量组成的数组
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valueOf方法，将字符串转为枚举对象(字符串为枚举常量名，类似于Python字典)，要求是字符串必须为已经有的常量名，否则会报异常
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compareTo 比较两个枚举常量，按照编号比较，返回编号差值 调用对象编号-比较对象编号
public class EnumMethod { public static void main(String[] args) { Season season = Season.AUTUMN; Season season1 = Season.AUTUMN; System.out.println(season.name()); System.out.println(season.ordinal()); Season[] values = Season.values(); for(Season value: values){ System.out.println(value); } Season season2 = Season.valueOf("SUMMER"); System.out.println("season2="+season2); System.out.println(Season.WINTER.compareTo(Season.SPRING)); } } //输出： AUTUMN 2 Season{name='春天', desc='温暖'} Season{name='夏天', desc='热'} Season{name='秋天', desc='凉爽'} Season{name='冬天', desc='冷'} season2=Season{name='夏天', desc='热'} 3
实现细节：
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使用enum关键字后就不能再继承其他类了，因为enum会隐式继承Enum，而Java是单继承机制
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枚举类和普通类一样，可以实现接口 例子：
public class EnumDetails { public static void main(String[] args) { Wants.CAT.cry(); } } interface Animal{ void cry(); } enum Wants implements Animal{ CAT,DOG,FISH,BIRD; @Override public void cry() { System.out.println("Cat"); } } //输出： Cat
注解(Annotation)
@注解名 可以理解为有功能的注释Java SE中使用较少，Java EE中大量使用
三个基本注解：
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@Override 重写父类方法，只能用于方法
会检查是否构成了重写，若没构成重写，会报错
打开源码会发现有一个注解@Target，这个注解是元注解，用于表示该注解是注解
class Father{ public void play(){ System.out.println("Father"); } } class Son extends Father{ //加不加@Override都可以构成重写，但是加了会对是否重写了进行检查 @Override public void play() { System.out.println("Son"); } } -
@Deprecated 用于表示某个程序元素(类或方法)已经过时(不推荐使用但是仍然可以使用)，可以修饰方法、类、字段、包、参数等(可以做版本升级的过渡使用)
public class AnnoTest { public static void main(String[] args) { A a = new A(); a.play(); } } @Deprecated class A{ @Deprecated public void play(){ System.out.println("A"); } }
作用域：方法、语句或者类上
元注解(了解即可，防止看源码的时候不知道这些注解是干什么的)
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@Retention //指定注解的作用范围(编译时、运行时…)
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@Target //指定注解在哪些地方可以使用
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@Documented //指定该注解是否会在javadoc体现
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@Inherited //子类会继承父类注解
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