list toMap

环境是jdk8以上使用lamdba表达式

public class ListToMap {
	public static void main(String[] args) {
		List<Book> bookList = new ArrayList<>();
		bookList.add(new Book("The Fellowship of the Ring", 1954, "0395489318"));
			bookList.add(new Book("The Two Towers", 1954, "0345339711"));
			bookList.add(new Book("The Return of the King", 1955, "0618129111"));
			Map<String,Book> bookMap=bookList.stream()
				.collect(Collectors.toMap(Book::getName, Function.identity()));
			System.out.println(bookMap);
	}
}
class Book {
	private String name;
	private int releaseYear;
	private String isbn;

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public int getReleaseYear() {
		return releaseYear;
	}

	public void setReleaseYear(int releaseYear) {
		this.releaseYear = releaseYear;
	}

	public String getIsbn() {
		return isbn;
	}

	public void setIsbn(String isbn) {
		this.isbn = isbn;
	}

	public Book(String name, int releaseYear, String isbn) {
		this.name = name;
		this.releaseYear = releaseYear;
		this.isbn = isbn;
	}

	@Override
	public String toString() {
		return "Book{" +
			"name='" + name + '\'' +
			", releaseYear=" + releaseYear +
			", isbn='" + isbn + '\'' +
			'}';
	}
}

以上代码输出

1
2

{The Return of the King=Book{name='The Return of the King', releaseYear=1955, isbn='0618129111'}, The Fellowship of the Ring=Book{name='The Fellowship of the Ring', releaseYear=1954, isbn='0395489318'}, The Two Towers=Book{name='The Two Towers', releaseYear=1954, isbn='0345339711'}}

解决key冲突

上面的例子运行得很好，但是如果有一个重复的key会发生什么呢?
抛出异常

1	Exception in thread "main" java.lang.IllegalStateException: Duplicate key Book{name='The Return of the King', releaseYear=1955, isbn='0618129111'}

要解决这个问题，我们需要使用另一种toMap()方法，附加一个参数，mergeFunction:

public static <T, K, U>
Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,
                                Function<? super T, ? extends U> valueMapper,
                                BinaryOperator<U> mergeFunction) {
    return toMap(keyMapper, valueMapper, mergeFunction, HashMap::new);
}

代码如下

1 2	Map<String,Book> bookMap=bookList.stream() .collect(Collectors.toMap(Book::getName, Function.identity(),(k,v)->k));

java自定义排序

List 接口sort方法实现Comparator 接口进行排序

需要注意有可能抛出：Comparison method violates its general contract异常

保证实现接口返回0 -1 1.
需要实现升序返回：-1 0 1
实现降序返回：1 0 -1

Java

Java

list toMap

解决key冲突

java自定义排序

timesort