Summary of usage of reference types in java

1. class as a member variable

When defining a class Role (game character), the code is as follows:

Use the int type to represent the character id and health value, and use the String type to represent the name. At this time, String itself is a reference type. Since it is used like a constant, its existence as a reference type is often ignored. If we continue to enrich the definition of this class and give the Role attributes such as equipment, how will we write it?

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Define weapon class, which will increase attack capability:

Define wearing armor class, which will increase defense ability, that is, increase health value:

Define character class:

Test class:

public class Test { public static void main(String[] args) { // 创建Weapon 对象 Weapon wp = new Weapon("屠龙刀", 999999); // 创建Armour 对象 Armour ar = new Armour("麒麟甲", 10000); // 创建Role 对象 Role r = new Role(); // 设置武器属性 r.setWeapon(wp); // 设置盔甲属性 r.setArmour(ar); // 攻 击 r.attack(); // 穿戴盔甲 r.wear(); } } 输出结果: 使用屠龙刀,造成999999点伤害 穿上麒麟甲 ,生命值增加10000

Tip: When a class is used as a member variable, assigning a value to it is actually assigning it an object of the class.

2. Interface as a member variable

Interface is an encapsulation of methods, corresponding to the game, it can be regarded as an extended game Character's skills. Therefore, if we want to expand more powerful skills, we can add interfaces as member variables in Role to set different skills.

Define the interface:

Define the role class:

Define test class:

public class Test { public static void main(String[] args) { // 创建游戏角色 Role role = new Role(); // 设置角色法术技能 role.setFaShuSkill(new FaShuSkill() { @Override public void faShuAttack() { System.out.println("纵横天下"); } }); // 发动法术攻击 role.faShuSkillAttack(); // 更换技能 role.setFaShuSkill(new FaShuSkill() { @Override public void faShuAttack() { System.out.println("逆转乾坤"); } }); // 发动法术攻击 role.faShuSkillAttack(); } } 输出结果: 发动法术攻击:纵横天下攻击完毕 发动法术攻击:逆转乾坤攻击完毕

Tip: We use an interface as a member variable to change skills at any time. This design is more flexible and enhances the expansion of the program. sex. When an interface is used as a member variable, assigning a value to it is actually assigning it a subclass object of the interface.

3. Interface as method parameter and return value type

When the interface is used as a method parameter, what needs to be passed? When an interface is used as the return value type of a method, what needs to be returned? Yes, in fact, they are all subclass objects of it.

We are not unfamiliar with the ArrayList class. Looking at the API, we find that, in fact, it is the implementation class of thejava.util.Listinterface. So, when we see the List interface as a parameter or return value type, of course we can pass or return an ArrayList object.

Please observe the following method: Get all the even numbers in a set.

Define method:

public static List getEvenNum(List list) { // 创建保存偶数的集合 ArrayList evenList = new ArrayList<>(); // 遍历集合list,判断元素为偶数,就添加到evenList中 for (int i = 0; i < list.size(); i++) { Integer integer = list.get(i); if (integer % 2 == 0) { evenList.add(integer); } } /* 返回偶数集合因为getEvenNum方法的返回值类型是List,而ArrayList是List的子类, 所以evenList可以返回 */ return evenList; }

Calling method:

## Tip: When the interface is used as a parameter, Pass its subclass object. When an interface is used as a return value type, its subclass object is returned.

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