b. More importantly, the methods in the interface can be modified with default and then add method bodies
2. Why can't we use the default method to override the equals, hashcode, and toString methods?
That is, the interface cannot provide a default implementation of any method of the Object class. If a class implements a method, that always takes precedence over the default implementation. Once all interface instances are subclasses of Object, all interface instances already have non-default implementations of equals/hashCode/toString. Therefore, a default version of these on the interface is useless and will not be compiled.
3. Functional interface
The core concept is functional interface. If an interface defines a single abstract method, then the interface becomes a functional interface. For example, java.lang.Runnable is a functional interface because it only defines one abstract method:
public abstract void run();
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What is a functional interface? There are two situations: 1. The interface has only one abstract method, abstract modification 2 .The interface has only one abstract method, abstract modification. At the same time, it contains multiple default methods, because the default method is modified by default, not abstract.
At the same time, a new Annotation is introduced: @FunctionalInterface. You can put it in front of an interface to indicate that the interface is a functional interface. Adding an interface to it won't compile unless you manage to turn it into a functional interface. It's a bit like @Override, which declares an intention to use it to prevent you from using it incorrectly.
4.Lambdas
A very valuable property of functional interfaces is that they can be instantiated with lambdas. Here are some examples of lambdas:
On the left is a comma-separated input list of the specified type, and on the right is a code block with return:
(int x, int y) -> { return x + y; }
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On the left is a comma-separated input list of the derived type , the right side is the return value:
(x, y) -> x + y
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The left side is a single parameter of the derivation type, the right side is a return value:
x -> x * x
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There is no input on the left side (official name: "burger arrow"), and it is returned on the right side A value:
() -> x
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The left side is a single parameter of the deduced type, and the right side is a code block with no return value (return void):
x -> { System.out.println(x); }
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Static method reference:
String::valueOf
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Non Static method reference:
Object::toString
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Inherited function reference:
x::toString
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Constructor reference:
ArrayList::new
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You can come up with some function reference formats that serve as shorthand for other lambda formats.
控制流程 (break, early return) -在上面的 forEach例子中，传统的继续方式有可能通过在lambda之内放置 "return;"来实现。但是，没有办法中断循环或者从lambda中通过包含方法的结果返回一个数值。例如：
final String secret = "foo"; boolean containsSecret(Iterable<String> values) {
values.forEach(s -> { if (secret.equals(s)) {
??? // want to end the loop and return true, but can't }});
}
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进一步阅读关于这些问题的资料，看看这篇Brian Goetz写的说明：在 Block中响应“已验证例外”What are the new features of Java8?
控制流程 (break, early return) -在上面的 forEach例子中，传统的继续方式有可能通过在lambda之内放置 "return;"来实现。但是，没有办法中断循环或者从lambda中通过包含方法的结果返回一个数值。例如：
final String secret = "foo"; boolean containsSecret(Iterable<String> values) {
values.forEach(s -> { if (secret.equals(s)) {
??? // want to end the loop and return true, but can't }});
}
Ordering<String> order = Ordering.from((a, b) -> ...);
CacheLoader<String, String> loader = CacheLoader.from((key) -> ...);
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要深入阅读，请参看由 Brian Goetz所做的说明： response to "Allow lambdas to implement abstract classes"。
What are the new features of Java8?
翻译于 4年前
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顶 翻译得不错哦！
java.util.function
Package summary: java.util.function
As an early proof of Comparator and Runnable, it has been defined in the JDK The interface happens to be compatible with lambdas expressions as a functional interface. In the same way you can define any functional interface or third-party library in your own code.
But there is a specific form of function interface, which is extensive and universal and did not exist in the previous JD card. A large number of interfaces have been added to the new java.util.function package. Here are some of them:
Function -T as input, returns R as output
Predicate -T as input, the returned boolean value as output
##Consumer -T as input, perform some action but no return value
Supplier - Without any input, return T
BinaryOperator -Two T as input, return one T as output, useful for "reduce" operation
These most primitive characteristics also exist. They are provided as int, long and double. For example:
IntConsumer - takes int as input, performs a certain action, and has no return value
There are some performance reasons here, mainly Interpretation avoids boxing and unboxing operations during input or output.
Waiting for PM Translated 4 years ago
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