了解Java中将方法保存到对象中

Question

In the javadoc there is an example of code in the class Pattern, which I do not understand regarding to the concept.

Pattern p = Pattern.compile("a*b");
Matcher m = p.matcher("aaaaab");
boolean b = m.matches();

• The compile method is static and gets saved into an object of the class Pattern. How does that work?
• The method matcher gets called from the object of the instance p and gets stored into a variable of the type Matcher. How does that work?

Answer 1

It's possible to look at the Java source code in your IDE / online . Thus, lets go through what's really happening here:

public static Pattern compile(String regex) {
    return new Pattern(regex, 0);
}

The static compile method doesn't actually compile or "save anything to the class", it just creates a new Pattern instance for you and returns it.

---

private Pattern(String p, int f) {
    //...
}

The Pattern class's constructor is private , meaning that you're not able to instantiate it yourself directly; meaning that you have to use Pattern.compile and friends.

---

Q: Why did they design it this way??

The main idea is that the constructor they've got going on is a bit weird --- it takes in that flags int, and constructors in other classes in the standard library are worse, to the point where you might need quite in-depth knowledge of the class's inner workings in order to use them. So, it can make sense to move those to being private implementation details, and then make a few methods which create your class correctly for all the use cases people need, and to be reasonably consistent with how you do it.