项目背景

在使用枚举的时候存在需要通过字符串来获取枚举的情况，当传参字段与枚举字段完全相同的情况下我们就可以使用Enum类中valueOf() 方法，不需要单独写一个方法

Enum valueOf

/**
   * 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.)
   **/
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);
}

我们可以看到valueOf方法其实就是传参name跟枚举常量一样时返回对应的枚举

使用案例

public enum TrafficType {
    Red("红灯"),
    Green("绿灯"),
    Yellow("黄灯");

    private String name;

    TrafficType(String name) {
        this.name = name;
    }
}

public class TrafficTypeTest {

    public static void main(String[] args) {
        System.out.println(TrafficType.valueOf("Red"));
        System.out.println("----");
        System.out.println(TrafficType.valueOf("red"));
    }
}

上面我定义了一个红绿灯的枚举，想要通过传值或者枚举，此时看下结果

Connected to the target VM, address: '127.0.0.1:62338', transport: 'socket'
Red
----
Disconnected from the target VM, address: '127.0.0.1:62338', transport: 'socket'
Exception in thread "main" java.lang.IllegalArgumentException: No enum constant test.TrafficType.red
    at java.lang.Enum.valueOf(Enum.java:238)
    at test.TrafficType.valueOf(TrafficType.java:7)
    at test.TrafficTypeTest.main(TrafficTypeTest.java:12)

Process finished with exit code 1

我们可以看到只有当传参跟枚举一样时才能返回正确的枚举，不然会报一个IllegalArgumentException的异常