Apple类与Orange类除了都是Object外没有任何共性，ArrayList保存的是Object，因此我们不但可以通过add()方法将Apple对象放入这个容器，还可以放Orange对象。
当使用get()方法获取ArrayList中的对象时，你认为你取出的是想要的Apple对象，实际上得到只是Object引用，必须将其强制转型为Apple对象才能调用Apple类的方法及属性。
如果一个类没有显示地声明继承自哪个类，那么它自动地继承Object
example：
//: holding/ApplesAndOrangesWithoutGenerics.java
// Simple container example (produces compiler warnings).
// {ThrowsException}
import java.util.*;
class Apple {
private static long counter;
private final long id = counter++;
public long id() { return id; }
}
class Orange {}
public class ApplesAndOrangesWithoutGenerics {
@SuppressWarnings("unchecked")
public static void main(String[] args) {
ArrayList apples = new ArrayList();
for(int i = 0; i < 3; i++)
apples.add(new Apple());
// Not prevented from adding an Orange to apples:
apples.add(new Orange());
for(int i = 0; i < apples.size(); i++)
// 强制转型为Apple，否则Object类并没有id()方法
((Apple)apples.get(i)).id();
// Orange is detected only at run time
}
定义用来保存Apple对象的ArrayList，可以预定义声明ArrayList<Apple>
尖括号括起来的是
类型参数，指定了这个容器实例可以保存的类型，通过使用泛型，可以在编译期防止将错误类型的对象放置到容器中。
example：
//: holding/ApplesAndOrangesWithGenerics.java
import java.util.*;
public class ApplesAndOrangesWithGenerics {
public static void main(String[] args) {
ArrayList<Apple> apples = new ArrayList<Apple>();
for(int i = 0; i < 3; i++)
apples.add(new Apple());
// Compile-time error:
// apples.add(new Orange());
for(int i = 0; i < apples.size(); i++)
// 此时不需要强制转型
System.out.println(apples.get(i).id());
// Using foreach:
for(Apple c : apples)
System.out.println(c.id());
}
} /* Output:
0
1
2
0
1
2
*///:~
List
List承诺可以将元素维护在特定的序列中
- 基本的ArrayList：用于随机访问元素，但是在List中间插入和移除元素时较慢
- LinkedList：通过较低代价在List中间进行插入和删除操作，但随机访问较慢
Collections.shuffle()方法
- Collections的静态方法，按照某随机源，随机打乱对象顺序，置换可能性相等
retainAll()方法
- x.retainAll(y) 取x与y的交集保留在x中，所产生的行为依赖于equals()方法
subList()方法
- 所产生的列表就是来自初始列表，任何子串的变化都会返回初始列表
- “子串永远是字串”
//: holding/ListFeatures.java
import typeinfo.pets.*;
import java.util.*;
import static net.mindview.util.Print.*;
public class ListFeatures {
public static void main(String[] args) {
Random rand = new Random(47);
List<Pet> pets = Pets.arrayList(7);
print("1: " + pets);
Hamster h = new Hamster();
pets.add(h); // Automatically resizes
print("2: " + pets);
print("3: " + pets.contains(h));
pets.remove(h); // Remove by object
Pet p = pets.get(2);
print("4: " + p + " " + pets.indexOf(p));
Pet cymric = new Cymric();
print("5: " + pets.indexOf(cymric));
print("6: " + pets.remove(cymric));
// Must be the exact object:
print("7: " + pets.remove(p));
print("8: " + pets);
pets.add(3, new Mouse()); // Insert at an index
print("9: " + pets);
List<Pet> sub = pets.subList(1, 4);
print("subList: " + sub);
print("10: " + pets.containsAll(sub));
Collections.sort(sub); // In-place sort
print("sorted subList: " + sub);
// Order is not important in containsAll():
print("11: " + pets.containsAll(sub));
Collections.shuffle(sub, rand); // Mix it up
print("shuffled subList: " + sub);
print("12: " + pets.containsAll(sub));
List<Pet> copy = new ArrayList<Pet>(pets);
sub = Arrays.asList(pets.get(1), pets.get(4));
print("sub: " + sub);
copy.retainAll(sub);
print("13: " + copy);
copy = new ArrayList<Pet>(pets); // Get a fresh copy
copy.remove(2); // Remove by index
print("14: " + copy);
copy.removeAll(sub); // Only removes exact objects
print("15: " + copy);
copy.set(1, new Mouse()); // Replace an element
print("16: " + copy);
copy.addAll(2, sub); // Insert a list in the middle
print("17: " + copy);
print("18: " + pets.isEmpty());
pets.clear(); // Remove all elements
print("19: " + pets);
print("20: " + pets.isEmpty());
pets.addAll(Pets.arrayList(4));
print("21: " + pets);
Object[] o = pets.toArray();
print("22: " + o[3]);
Pet[] pa = pets.toArray(new Pet[0]);
print("23: " + pa[3].id());
}
} /* Output:
1: [Rat, Manx, Cymric, Mutt, Pug, Cymric, Pug]
2: [Rat, Manx, Cymric, Mutt, Pug, Cymric, Pug, Hamster]
3: true
4: Cymric 2
5: -1
6: false
7: true
8: [Rat, Manx, Mutt, Pug, Cymric, Pug]
9: [Rat, Manx, Mutt, Mouse, Pug, Cymric, Pug]
subList: [Manx, Mutt, Mouse]
10: true
sorted subList: [Manx, Mouse, Mutt]
11: true
shuffled subList: [Mouse, Manx, Mutt]
12: true
sub: [Mouse, Pug]
13: [Mouse, Pug]
14: [Rat, Mouse, Mutt, Pug, Cymric, Pug]
15: [Rat, Mutt, Cymric, Pug]
16: [Rat, Mouse, Cymric, Pug]
17: [Rat, Mouse, Mouse, Pug, Cymric, Pug]
18: false
19: []
20: true
21: [Manx, Cymric, Rat, EgyptianMau]
22: EgyptianMau
23: 14
*///:~
Refer
《Java编程思想 第4版》
Logging
- 20190922 xiaomi init