内存和静态

不同于C/C++的内存四区，Java有自己的内存管理模型，而本文将介绍一部分有关内存的问题。

new关键字

new关键字是在堆区开辟出一块内存用于储存数据。
所有通过new关键字而创建出来的都被储存在堆区。
对于同一个字符串phil616

String str1 = new String("phil616");
String str2 = "phil616"

二者一个是在公共池中创建，一个是在堆区创建，所以使用逻辑运算符 == 判断二者是否相等是不正确的。
因为一个是属性，而另一个是对象。二者的本质不同。

static关键字

static表示在内存中只有一份。
如果两个对象共用一个static变量或方法，那么这个所有使用这个static变量的对象都会更改。
也就是说，我虽然使用同一个类创建了两个对象，但是对象使用的是同一个static属性，如果上一个对象对static属性进行更改，那么下一个也会跟着更改。因为二者公用一份内存，公用同一个变量。
但是这样的好处是大大节省了内存开销。尤其是工具类方法，例如get或set，可以大大节省内存。

package com.yuque.phil616.memorys;
public class MainDemo {
    /*
    * key word 'static' means this function/method/variable/property only exits
    * one in memory, to understand the meaning of memory, we need to know what
    * does the memory consist.
    * There are four areas in memory
    * heap and thread stack. we don't need to know what are them for now.
    * however, static means there are only one copy of it in memory.
    * */
    public static void main(String[] args) {
        Mer m1 = new Mer();
        Mer m2 = new Mer();
        m1.setI(5);
        m2.setI(10);
        System.out.println(m1.getI());
        System.out.println(m2.getI());
        /*
        * We can find out that they are both 10, because they are using a same
        * block to storage i. in this case, after I set i by 5, the value of i
        * changed into 5, and I set i by 10, it have changed into 10;
        * */
    }
    /*
    * for example, this variable/property i ara static, means there are only one
    * space to stored this variable.
    * */
    static int i = 10;
    //If you want to understand more about memory in C/C++
    //Please access passage https://zhuanlan.zhihu.com/p/350301516
}
class Mer{
    static int i;
    /*
    * as the same reason, if a method of function is using like a tool,
    * just modify them to a static method, this made objects smaller.
    * */
    public static void setI(int i) {
        Mer.i = i;
    }
    public static int getI() {
        return i;
    }
}