一开始跟着视频有点绕，干脆把每个版本都试一遍会稍微清晰一些；

Version1: error

Dog.java

public class Dog {public String name;private int size;public Dog(String n, int s) {name = n;size = s;}public void bark() {System.out.println(name + " says: bark");}
}

Maxmizer.java

public class Maximizer {public static Object max(Object[] items) {int maxDex = 0;for (int i = 0; i < items.length; i += 1) {if (items[i] > items[maxDex]) {  //报错：Operator '>' cannot be applied to 'java.lang.Object', 'java.lang.Object'maxDex = i;}}return items[maxDex];}
}

DogLauncher.java

public class DogLauncher {public static void main(String[] args) {Dog d1 = new Dog("Elyse", 3);Dog d2 = new Dog("Sture", 9);Dog d3 = new Dog("Benjamin", 15);Dog[] dogs = new Dog[]{d1, d2, d3};Dog maxDog = (Dog) Maximizer.max(dogs);maxDog.bark();}
}

Version2: Dog class＋maxDog method

Dog.java

加了一个maxDog method：可以暂时缓解，但是难道每一个animal我都要写一个maxbrbrbr method吗？重复性好强！要改！

public class Dog {public String name;private int size;public Dog(String n, int s) {name = n;size = s;}public void bark() {System.out.println(name + " says: bark");}// vs version1: add maxDog methodpublic static Dog maxDog(Dog[] dogs) {  if (dogs == null || dogs.length == 0) {return null; }Dog maxDog = dogs[0];for (Dog d : dogs) {if (d.size > maxDog.size) {maxDog = d;            }}return maxDog;}
}

Maxmizer.java (这个version不需要)
DogLauncher.java

public class DogLauncher {public static void main(String[] args) {Dog d1 = new Dog("Elyse", 3);Dog d2 = new Dog("Sture", 9);Dog d3 = new Dog("Benjamin", 15);Dog[] dogs = new Dog[]{d1, d2, d3};Dog maxDog = Dog.maxDog(dogs);  // 和version1比这里变了 同时体会了一把static method的callmaxDog.bark();}
}

Version3: OurComparable Interface

这个version主要是新建了一个interface：guarantee a comparision method.

Interface inheritance says what a class can do, in this case compare.

Dog.java

public class Dog implements OurComparable{  //改public String name;private int size;public Dog(String n, int s) {name = n;size = s;}public void bark() {System.out.println(name + " says: bark");}@Override  // 改public int compareTo(OurComparable obj) {Dog cmpDog = (Dog) obj;return this.size - cmpDog.size;}
}

Maxmizer.java

public class Maximizer {public static OurComparable max(OurComparable[] items) {  // 改int maxDex = 0;for (int i = 0; i < items.length; i += 1) {int cmp = items[i].compareTo(items[maxDex]);        // 改if (cmp > 0) {                                      // 改maxDex = i;}}return items[maxDex];}
}

DogLauncher.java

public class DogLauncher {public static void main(String[] args) {Dog d1 = new Dog("Elyse", 3);Dog d2 = new Dog("Sture", 9);Dog d3 = new Dog("Benjamin", 15);Dog[] dogs = new Dog[]{d1, d2, d3};Dog maxDog = (Dog) Maximizer.max(dogs);  // 改maxDog.bark();}
}

OurComparable.java

// 改
public interface OurComparable {int compareTo(OurComparable obj);  // int compareTo(Object obj); is also ok, no meaningful difference;
}

Version4: Comparable | built-in interface

Dog.java
可以看到解决了一部分Awkward casting to/from Objects的问题；

public class Dog implements Comparable<Dog>{public String name;private int size;public Dog(String n, int s) {name = n;size = s;}public void bark() {System.out.println(name + " says: bark");}@Override // 改public int compareTo(Dog cmpDog) {return this.size - cmpDog.size;}
}

Maxmizer.java

public class Maximizer {public static Comparable max(Comparable[] items) {  //改int maxDex = 0;for (int i = 0; i < items.length; i += 1) {int cmp = items[i].compareTo(items[maxDex]);if (cmp > 0) {maxDex = i;}}return items[maxDex];}
}

DogLauncher.java | 没变

public class DogLauncher {public static void main(String[] args) {Dog d1 = new Dog("Elyse", 3);Dog d2 = new Dog("Sture", 9);Dog d3 = new Dog("Benjamin", 15);Dog[] dogs = new Dog[]{d1, d2, d3};Dog maxDog = (Dog) Maximizer.max(dogs);maxDog.bark();}
}

Comparable.java
Built-in interface

Lots of built in classes implement Comparable (e.g. String).
Lots of libraries use the Comparable interface (e.g. Arrays.sort).
Avoids need for casts.

// 改
public interface Comparable<T> {public int compareTo(T obj);
}

Version5: Comparators

5.1

Dog.java

public class Dog implements Comparable<Dog>{public String name;private int size;public Dog(String n, int s) {name = n;size = s;}public void bark() {System.out.println(name + " says: bark");}@Overridepublic int compareTo(Dog cmpDog) {return this.size - cmpDog.size;}// 改public static class NameComparator implements Comparator<Dog> {public int compare(Dog d1, Dog d2) {return d1.name.compareTo(d2.name);}}
}

Maxmizer.java comparator本身不需要，但是comparable需要
DogLauncher.java

public class DogLauncher {public static void main(String[] args) {Dog d1 = new Dog("Elyse", 3);Dog d2 = new Dog("Sture", 9);Dog d3 = new Dog("Benjamin", 15);Dog[] dogs = new Dog[]{d1, d2, d3};Dog maxDog = (Dog) Maximizer.max(dogs);maxDog.bark();// 改Comparator<Dog> cd = new Dog.NameComparator();if (cd.compare(d1, d3) > 0) {d1.bark();} else {d3.bark();}}
}

Comparator.java

public interface Comparator<T> {int compare(T o1, T o2);
}

5.2

在5.1的基础上改了

Dog.java

public class Dog implements Comparable<Dog>{public String name;private int size;public Dog(String n, int s) {name = n;size = s;}public void bark() {System.out.println(name + " says: bark");}@Overridepublic int compareTo(Dog cmpDog) {return this.size - cmpDog.size;}private static class NameComparator implements Comparator<Dog> { // 改public int compare(Dog d1, Dog d2) {return d1.name.compareTo(d2.name);}}// 改public static Comparator<Dog> getNameComparator() {return new NameComparator();}
}

DogLauncher.java

public class DogLauncher {public static void main(String[] args) {Dog d1 = new Dog("Elyse", 3);Dog d2 = new Dog("Sture", 9);Dog d3 = new Dog("Benjamin", 15);Dog[] dogs = new Dog[]{d1, d2, d3};Dog maxDog = (Dog) Maximizer.max(dogs);maxDog.bark();Comparator<Dog> cd = Dog.getNameComparator();   // 改if (cd.compare(d1, d3) > 0) {d1.bark();} else {d3.bark();}}
}

Version6: self-try

写成
- Comparator:
  1. SizeCompare();
  2. NameCompare();

Dog.java

public class Dog implements Comparable<Dog>{public String name;private int size;public Dog(String n, int s) {name = n;size = s;}public void bark() {System.out.println(name + " says: bark");}@Overridepublic int compareTo(Dog cmpDog) {return this.size - cmpDog.size;}private static class SizeComparator implements Comparator<Dog> {@Overridepublic int compare(Dog d1, Dog d2) {return d1.size - d2.size;}}public static Comparator<Dog> getSizeComparator() {return new SizeComparator();}private static class NameComparator implements Comparator<Dog> {@Overridepublic int compare(Dog d1, Dog d2) {return d1.name.compareTo(d2.name);}}public static Comparator<Dog> getNameComparator() {return new NameComparator();}
}

Maxmizer.java

public class Maximizer {public static Comparable max(Comparable[] items) {int maxDex = 0;for (int i = 0; i < items.length; i += 1) {int cmp = items[i].compareTo(items[maxDex]);if (cmp > 0) {maxDex = i;}}return items[maxDex];}public static Comparable max(Comparable[] items, Comparator cd) {int maxDex = 0;for (int i = 0; i < items.length; i += 1) {int cmp = cd.compare(items[i], items[maxDex]);if (cmp > 0) {maxDex = i;}}return items[maxDex];}
}

DogLauncher.java

public class DogLauncher {public static void main(String[] args) {Dog d1 = new Dog("Elyse", 3);Dog d2 = new Dog("Sture", 9);Dog d3 = new Dog("Benjamin", 15);Dog[] dogs = new Dog[]{d1, d2, d3};Dog maxDog = (Dog) Maximizer.max(dogs);maxDog.bark();Comparator<Dog> nc = Dog.getNameComparator();Dog maxDog2 = (Dog) Maximizer.max(dogs, nc);maxDog2.bark();Comparator<Dog> sc = Dog.getSizeComparator();Dog maxDog3 = (Dog) Maximizer.max(dogs, sc);maxDog3.bark();}
}

Comparable.java

public interface Comparable<T> {public int compareTo(T obj);
}

Comparator.java

public interface Comparator<T> {int compare(T o1, T o2);
}

recap

Interfaces provide us with the ability to make callbacks.
In Java, we do this by wrapping up the needed function in an interface (e.g. Arrays.sort needs compare which lives inside the comparator interface).

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Version1: error

Version2: Dog class＋maxDog method

Version3: OurComparable Interface

Version4: Comparable | built-in interface

Version5: Comparators

5.1

5.2

Version6: self-try

recap

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