How to use the TreeSet function in Java for ordered set operations

Java's TreeSet is an ordered set implemented based on red-black trees. Its feature is that elements are sorted in order of size, and elements can be quickly added, deleted and searched. This article will introduce how to use the TreeSet function in Java to perform ordered set operations so that it can be better applied in actual programming.

1. Basic operations of TreeSet

1. Create a TreeSet object

To use TreeSet, you need to create a TreeSet object first. You can use the parameterless constructor to create an empty TreeSet, or you can specify a Comparator to customize the collation when creating the TreeSet object.

Sample code:

TreeSet<Integer> set = new TreeSet<>();
TreeSet<String> set2 = new TreeSet<>(new Comparator<String>() {
    @Override
    public int compare(String o1, String o2) {
        // 自定义排序规则
        return o1.compareToIgnoreCase(o2);
    }
});

2. Add elements

Add elements to TreeSet through the add() method. TreeSet will automatically sort elements in order of size, and the same elements will only be saved once.

Sample code:

set.add(3);
set.add(1);
set.add(2);
set.add(3);
// 结果为[1, 2, 3]
System.out.println(set);

3. Delete elements

You can use the remove() method to delete elements in the TreeSet.

Sample code:

set.remove(3);
// 结果为[1, 2]
System.out.println(set);

4. Determine whether an element exists

You can use the contains() method to determine whether an element exists in the TreeSet.

Sample code:

boolean contains = set.contains(2);
// 结果为true
System.out.println(contains);

5. Get the number of elements

You can use the size() method to get the number of elements in the TreeSet.

Sample code:

int size = set.size();
// 结果为2
System.out.println(size);

6. Traverse elements

You can use the for loop or forEach() method to traverse the elements in the TreeSet.

Sample code:

for (Integer i : set) {
    System.out.print(i + " ");
}
System.out.println();

set.forEach(System.out::println);

Output result:

1 2 
1
2

2. Advanced operations of TreeSet

1. Get the first element and the last element

You can use the first() and last() methods to get the first element and the last element in the TreeSet.

Sample code:

Integer first = set.first();
Integer last = set.last();
// 结果为1 2
System.out.println(first + " " + last);

2. Obtain sub-collection

You can use the subSet() method to obtain a sub-collection of TreeSet, which contains fromElement (including) to elements between toElement (exclusive). If fromElement is not specified, it means starting from the first element in the TreeSet.

Sample code:

TreeSet<Integer> subSet = (TreeSet<Integer>) set.subSet(1, 2);
// 结果为[1]
System.out.println(subSet);

TreeSet<String> subSet2 = (TreeSet<String>) set2.subSet("A", "c");
// 结果为[b, C]
System.out.println(subSet2);

It should be noted that if the subcollection changes, the original TreeSet will also change.

3. Get the sub-collection of head or tail elements

You can use the headSet() method and tailSet() method to get the head (excluding toElement) or tail (including fromElement) of the TreeSet A subcollection of elements.

Sample code:

TreeSet<Integer> headSet = (TreeSet<Integer>) set.headSet(2);
// 结果为[1]
System.out.println(headSet);

TreeSet<String> tailSet = (TreeSet<String>) set2.tailSet("b");
// 结果为[b, C]
System.out.println(tailSet);

It should also be noted that if the subcollection changes, the original TreeSet will also change.

4. Get elements that are smaller or larger than the specified element

You can use the lower() method, floor() method, higher() method and ceiling() method to get the element that is smaller or larger than the specified element. Big elements.

Sample code:

Integer lower = set.lower(2);
Integer floor = set.floor(2);
Integer higher = set.higher(1);
Integer ceiling = set.ceiling(1);
// 结果为1 2 2 1
System.out.println(lower + " " + floor + " " + higher + " " + ceiling);

It should be noted that:

• The difference between the lower() method and the floor() method is: the lower() method obtains the specified ratio The largest element with a smaller element is the largest element that is less than the specified element; and the floor() method obtains the nearest element that is smaller than the specified element, that is, the nearest element that is less than or equal to the specified element.
• The difference between the higher() method and the ceiling() method is: the higher() method obtains the smallest element that is larger than the specified element, that is, the smallest element that is larger than the specified element; while the ceiling() method obtains the smallest element that is larger than the specified element. The nearest element that is greater than or equal to the specified element.

3. Summary

This article introduces the basic operations and advanced operations of the TreeSet function in Java. Corresponding operation methods can be selected according to actual needs to perform ordered collection operations. It should be noted that when using the subSet() method, headSet() method and tailSet() method, if the subset changes, the original TreeSet will also change, so you need to keep this in mind.

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