java

Groovy学习

sort方法作用于Set和List时的区别

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def a = new HashSet()
a << [3, 1, 2]

def b = new ArrayList()
b << [2, 1, 3]

assert b.sort().is(b) //true
assert a.sort().is(a) //false

sum and flatten 作用于Collection时的区别

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[].sum() returns null
[].flatten() returns []
//原因在于调用sum时默认initvalue为null,当list为空时直接返回initvalue
private static Object sum(Iterable self, Object initialValue, boolean first) {
    Object result = initialValue;
    Object[] param = new Object[1];
    for (Object next : self) {
        param[0] = next;
        if (first) {
            result = param[0];
            first = false;
            continue;
        }
        MetaClass metaClass = InvokerHelper.getMetaClass(result);
        result = metaClass.invokeMethod(result, "plus", param);
    }
    return result;
}

@EqualsAndHashCode注解

当exclude类所有field及property时，会导制所有实例都相等

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@EqualsAndHashCode(excludes = ['x', 'y'])
static class Point {
    int x
    int y
}

public static void main(String[] args) {
    def point0 = new Point(x: 1, y: 1)
    def point1 = new Point(x: 2, y: 2)
    assert point0 == point1
}

对ORM类慎用该注解，可能会导制关联丢失，当mutable字段变化时，会导制hash值改变，导制无法定位到槽位，

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@EqualsAndHashCode
static class Point {
    int x
    int y
}

public static void main(String[] args) {
    def point0 = new Point(x: 1, y: 1)
    def point1 = new Point(x: 2, y: 2)
    def set = new HashSet()
    set << point0
    set << point1

    assert set.contains(point0)

    point0.x = 6
    assert set.contains(point0)  //false !!!
}