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Although Java as a compilation language, but it also provides a lot of runtime capabilities, today introduces a very basic knowledge, variable parameter passing

Creating a Map object is quite common in daily development. Now I want to write a utility class that makes it very easy to create and initialize a Map object

So we can implement a MapUtil utility class to support this scenario

 public static <K, V> Map<K, V> newMap(K k, V v, Object... kv) {
        Map<K, V> ans = new HashMap<>();
        ans.put(k, v);
        for (int i = 0; i < kv.length; i += 2) {
            ans.put((K) kv[i], (V) kv[1]);
        }
        return ans;
}
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Note that kv is the variable argument we are talking about. Inside the method, kv can be treated as an array object (and a safe object, not null when not passed).

Here are some things to be aware of when using mutable parameters

Variable arguments conflict with array arguments

Note that the following two methods do not occur at the same time, and directly cause a compilation error

public static <K, V> Map<K, V> newMap(K k, V v, Object... kv)
public static <K, V> Map<K, V> newMap(K k, V v, Object[] kv)
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Overload selection

If the method has only one variable argument, newMap(“key”, “value”) will not give an error and will directly access the following method, with kv as an empty array

public static <K, V> Map<K, V> newMap(K k, V v, Object... kv)
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When an overload occurs, it is as follows

public static <K, V> Map<K, V> newMap(K k, V v, Object... kv)
public static <K, V> Map<K, V> newMap(K k, V v)
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Which of the above two methods will be called if only two parameters are passed?

  • newMap("key", "value")The following method is called
  • Above method called by ‘newMap(“key”, “value”, “k”, “v”)

What happens when you pass a variable argument to an array

Although we use it as an array, is it possible to pass it as an array?

public static <K, V> Map<K, V> newMap(K k, V v, Object... kv) {
	Map<K, V> ans = new HashMap<>();
	ans.put(k, v);
	for (int i = 0; i < kv.length; i += 2) {
		ans.put((K) kv[i], (V) kv[1]);
	}
	return ans;
}

@Test
public void tt(a) {
	Map map = newMap("key"."value".new Object[]{"1"."2"});
	System.out.println(map);
}
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The actual output is as follows

{1=2, key=value}
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From a practical test, passing arrays is fine

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