Objects

为了防止该类被实例化，构造器私有化，并在内部抛出一个异常(经典)

private Objects() {
    throw new AssertionError("No java.util.Objects instances for you");

public static boolean equals(Object a, Object b)

如果两个对象是同一个实例或两个对象都为null，则返回true

如果一个对象为null，一个对象不为null,则返回false

到最后会返回a.equals(b)方法的返回值

public static boolean equals(Object a, Object b){
    return (a == b) || (a != null && a.equals(b));
}

public static boolean deepEquals(Object a, Object b)
检测两个对象是否深度相等

public static int hashCode(Object o)
如果为null返回0

public static int hashCode(Object o){
    return o == null ? 0 : o.hashCode();
}

public static int hash(Object... values) 返回一组对象的hash值，如果参数为null返回0,如果参数为空数组返回1

public static int hash(Object... values){
    return Arrays.hashCode(values);
}

public static String toString(Object o) 如果参数为null返回"null"，否则返回o.toString()

public static String toString(Object o){
    return String.valueOf(o);
}
//String.valueOf(Object obj)方法实现
public static String valueOf(Object obj){
    return (obj == null) ? "null" : obj.toString();
}

public static String toString(Object o, String nullDefault) 如果o为null返回nullDefault。否则返回o.toString()

public static String toString(Object o, String nullDefault){
    return (o == null)? nullDefault : o.toString();
}

public static T requireNonNull(T obj) 如果为null 抛出NullPointer异常

public static <T> T requireNonNull(T obj){
    if(obj == null){
        throw new NullPointerException();
    }
    return obj;

public static T requireNonNull(T obj, String message) 如果为null 抛出message的异常

public static <T> T requireNonNull(T obj, String message){
    if(obj == null){
        throw new NullPointerException(message);
    }
    retrun obj;   
}

public static boolean isNull(Object obj) 判断此指定参数是否为null

public static boolean isNull(Object obj){
    return obj == null;
}

public static boolean nonNull(Object obj) 判断指定参数是否非null

public static boolean nonNull(Object obj){
    return obj != null;
}

public static int compare(T a, T b, Comparator<? super T> c

public static <T> int compare(T a, T b, Comparator<? super T> c){
    return (a == b) ? 0 : c.compare(a, b);
}