如果 Map 则将元素添加到 map <string , stack<integer> &gt; 映射…在 java</string>

Question

I have a Map collection which maps String to Stack<Integer how can I put my stacks to this map? I've tried this so far but couldn't succeed.

Map<String,Stack<Integer>> map=new HashMap<>();
map.put("abc",new Stack<Integer>().push(123));

Answer 1

You can use computeIfAbsent() method which was added in Java 8:

Map<String, Stack<Integer>> map=new HashMap<>();
map.computeIfAbsent("abc", k -> new Stack<>()).push(123);

Answer 2

Since Stack#push method returns an Integer here. You can modify your code this way to achieve it:

Map<String, Stack<Integer>> map=new HashMap<>();
Stack<Integer> stack = new Stack<>();
stack.push(123);
map.put("abc", stack);

Update:

After looking at your comment on my answer. I think you want to accomplish something like this:

Map<String, Deque<Integer>> map = new HashMap<>();
Deque<Integer> deque = new ArrayDeque<>(); // use Deque instead of Stack
map.put("abc", deque); // putting the Deque in Map
map.get("abc").add(12);
map.get("abc").add(34);
map.get("abc").add(56);
map.get("abc").add(78);

System.out.println("Before removing: " + map);
map.get("abc").remove(); // removing first element
System.out.println("After  removing: " + map);

Output:

Before removing: {abc=[12, 34, 56, 78]}
After  removing: {abc=[34, 56, 78]}

Answer 3

Your implementation is wrong here. You have to create object first of type Stack<Integer> . As you have specified Stack<Integer> as values you should have to create Object of that type. Return type of push is of type itself so it cause error in your case.

    Map<String,Stack<Integer>> map=new HashMap<>();
    Stack<Integer> stack = new Stack<Integer>();
    stack.push(123);
    map.put("abc",stack);

Answer 4

First you need to fetch the stack, that you want to update, then you need to update the stack with the new element, and finally use the property of map, that replaces the elements, of the map already exists.

Stack<Integer> st = map.getOrDefault("abc",new Stack<Integer>());
st.push(123);
map.put("abc",st);

Answer 5

use this:

BiFunction<Stack<Integer>, Integer, Stack<Integer>> bi = (s, i) -> {
        s.push(i);
        return s;
    };

    map.put("abc", bi.apply(map.get("abc"), 134));