如何理解用Java连接在一起的创建类或方法的概念

Question

I want to know the concept of inner methods called one time example in Java.

Object ob = new Object().toString().charAt(0);

My question:

Are toString() and charAt(0); are classes or methods joined together using dot?

I confused how to ask these question if you understand my bad grammar please help me.

Answer 1

Each method of those returns a object, you are just chaining the call in the next object.

The equivalent of

char c = new Object().toString().charAt(0);

is the following code:

Object obj = new Object();
String objectStr = obj.toString();
char c = objectStr.charAt(0);

Answer 2

It is called "fluent interfaces" or method chaining. We can achive this with returning of object itself. Fluent apis used everywhere basic example of it is Builder classes.

In Java, Optional class or stream api is an example of fluent apis.

Here is an example builder class with fluend methods:

class ObjectBuilder {
    private SomeObject someObject = new SomeObject();

    public ObjectBuilder withPropertyX(int x) {
        someObject.setPropertyX(x);
        return this;
    }

    public ObjectBuilder withPropertyY(String y) {
        someObject.setPropertyY(y);
        return this;
    }

    public SomeObject build() {
        return someObject;
    }

}

ObjectBuilder builder = new ObjectBuilder();

SomeObject someObject = builder.withPropertyX(5).withPropertyY("test").build();

Another example with stream api:

List<String> students = new ArrayList();

    students.add("alice");
    students.add("jack");
    students.add("john");

    students.stream()
            .map(String::toUpperCase)
            .forEach(System.out::println);