JAVA小白的学习记录.8

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JAVA小白的学习记录.8

2023-03-28 22:53| 来源: 网络整理| 查看: 265

1.day181.1List集合中存自定义对象

现在集合容器中存数据的时候，字符串整型等

但是也可以存对象，集合中带有泛型是一个类

package com.qf.b_list; import java.util.ArrayList; import java.util.List; class Person { private String name; private int age; public Person() { } public Person(String name, int age) { this.name = name; this.age = age; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } @Override//重写的toString方法 Object类 //将内存地址转为一个字符串表示的形式可以看到打印的数据不再是一个内存地址了 //而是一个字符串了 public String toString() { return "[name:" + name + ", age:" + age + "]"; } } public class Demo1 { public static void main(String[] args) { List persons = new ArrayList(); Person person1 = new Person("狗蛋", 24); Person person2 = new Person("张三", 14); Person person3 = new Person("王五", 34); persons.add(person1); persons.add(person2); persons.add(person3); System.out.println(persons); //以上在存数据 //可以取数据 // System.out.println(persons.get(0));//获取的是集合中第一个对象 // System.out.println(persons.get(1)); // System.out.println(persons.get(2)); // // //想只把狗蛋取出来 // //先取出来第一个person对象 // System.out.println(persons.get(0).getName()); for (Person person : persons) { System.out.println(person.getName() + ":" + person.getAge()); } } } 1.2ArrayList源码

ArrayList底层是数组

RandomAccess :这个接口可以让ArrayList拥有快速随机访问的能力

源码： for循环比迭代器速度更快的

package com.qf.c_arrayList; import java.awt.List; import java.util.ArrayList; import java.util.ListIterator; public class Demo1 { public static void main(String[] args) { ArrayList list = new ArrayList(); for (int i = 0; i < 1000000; i++) { list.add("a"); } //更快点 long start = System.currentTimeMillis(); for (int i = 0; i < list.size(); i++) { System.out.println(list.get(i)); } long end = System.currentTimeMillis();//12018毫秒 System.out.println(end - start); // ListIterator listIterator = list.listIterator(); // while (listIterator.hasNext()) { // System.out.println(listIterator.next()); // }//15075 } } /** * Default initial capacity. */ // DEFAULT_CAPACITY 默认的容量= 10 private static final int DEFAULT_CAPACITY = 10; /** * Shared empty array instance used for empty instances. */ //空的数组 Object[] private static final Object[] EMPTY_ELEMENTDATA = {}; /** * Shared empty array instance used for default sized empty instances. We * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when * first element is added. */ //m默认的构造方法中空数组 private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; /** * The array buffer into which the elements of the ArrayList are stored. * The capacity of the ArrayList is the length of this array buffer. Any * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA * will be expanded to DEFAULT_CAPACITY when the first element is added. */ //真正存储元素的数组Object[] elementData transient Object[] elementData; // non-private to simplify nested class access /** * The size of the ArrayList (the number of elements it contains). * * @serial */ //最大数组的容量 private int size;

集合默认的可以存10个

/** * Constructs an empty list with an initial capacity of ten. */ public ArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; }

构造方法是干嘛的？ new ArrayList();

elementData 数组默认是10

public ArrayList(int initialCapacity) { if (initialCapacity > 0) { this.elementData = new Object[initialCapacity]; } else if (initialCapacity == 0) { this.elementData = EMPTY_ELEMENTDATA; } else { throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity); } }

new ArrayList(4);

public ArrayList(Collection

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JAVA小白的学习记录.8

JAVA小白的学习记录.8

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