2.2.1 添加元素
添加元素对象到当前集合中：

boolean add(E e);

添加另一个集合中的所有元素到当前集合中：

boolean addAll(Collection<? extends E> c);

示例：

package com.github.demo1;
import java.util.ArrayList;
import java.util.Collection;
/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class Test {
    public static void main(String[] args) {
        Collection<String> collection = new ArrayList<>();
        collection.add("hello");
        collection.add("world");
        System.out.println("collection = " + collection); // collection = [hello, world]
    }
}

示例：

package com.github.demo2;
import java.util.ArrayList;
import java.util.Collection;
/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class Test {
    public static void main(String[] args) {
        Collection<String> c1 = new ArrayList<>();
        c1.add("aa");
        c1.add("bb");
        c1.add("cc");
        Collection<String> c2 = new ArrayList<>();
        c2.add("ee");
        c2.add("ff");
        // 将c2中的所有元素添加到c1中
        c1.addAll(c2);
        System.out.println("c1 = " + c1); // c1 = [aa, bb, cc, ee, ff]
    }
}

示例：

package com.github.demo3;
import java.util.ArrayList;
import java.util.Collection;
/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class Test {
    public static void main(String[] args) {
        Collection<Object> c1 = new ArrayList<>();
        c1.add("aa");
        c1.add("bb");
        c1.add("cc");
        Collection<Object> c2 = new ArrayList<>();
        c2.add("ee");
        c2.add("ff");
        c1.add(c2);
        System.out.println("c1 = " + c1); // c1 = [aa, bb, cc, [ee, ff]]
    }
}

2.2.2 删除元素

• 从当前集合中删除第一个找到的与 obj 对象 equals 返回 true 的元素：

boolean remove(Object o);

从当前集合中删除所有与 coll 集合中相同的元素，即 this = this - this ∩ coll ：

boolean removeAll(Collection<?> coll)

清空集合：

void clear();

删除满足指定条件的集合中所有元素：

default boolean removeIf(Predicate<? super E> filter) {
        Objects.requireNonNull(filter);
        boolean removed = false;
        final Iterator<E> each = iterator();
        while (each.hasNext()) {
            if (filter.test(each.next())) {
                each.remove();
                removed = true;
            }
        }
        return removed;
}

示例：

package com.github.demo4;
import java.util.ArrayList;
import java.util.Collection;
/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class Test {
    public static void main(String[] args) {
        Collection<String> c1 = new ArrayList<>();
        c1.add("aa");
        c1.add("bb");
        c1.add("cc");
        System.out.println("c1 = " + c1); // c1 = [aa, bb, cc]
        c1.remove("aa");
        System.out.println("c1 = " + c1); // c1 = [bb, cc]
    }
}

示例：

package com.github.demo5;
import java.util.ArrayList;
import java.util.Collection;
/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class Test {
    public static void main(String[] args) {
        Collection<Object> c1 = new ArrayList<>();
        c1.add("aa");
        c1.add("bb");
        c1.add("cc");
        Collection<Object> c2 = new ArrayList<>();
        c2.add("ee");
        c2.add("ff");
        c1.add(c2);
        System.out.println("c1 = " + c1); // c1 = [aa, bb, cc]
        c1.remove(c2);
        System.out.println("c1 = " + c1); // c1 = [aa, bb, cc]
    }
}

示例：

package com.github.demo6;
import java.util.ArrayList;
import java.util.Collection;
/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class Test {
    public static void main(String[] args) {
        Collection<String> c1 = new ArrayList<>();
        c1.add("aa");
        c1.add("bb");
        c1.add("cc");
        Collection<String> c2 = new ArrayList<>();
        c2.add("ee");
        c2.add("ff");
        c1.addAll(c2);
        System.out.println("c1 = " + c1); // c1 = [aa, bb, cc, ee, ff]
        c1.removeAll(c2);
        System.out.println("c1 = " + c1); // c1 = [aa, bb, cc]
    }
}

示例：

package com.github.day11.collection1;
import org.junit.jupiter.api.Test;
import java.util.ArrayList;
import java.util.Collection;
/**
 * @author maxiaoke.com
 * @version 1.0
 * @since 2023-01-25 23:02:42
 */
public class CollectionTest {
    /**
     * 删除满足指定条件的此集合中的所有元素
     */
    @Test
    public void test06() {
        Collection<Integer> col = new ArrayList<>();
        col.add(1);
        col.add(2);
        col.add(3);
        col.add(4);
        // 删除 >2 的元素
        col.removeIf((i) -> i > 2);
        System.out.println("col = " + col);// col = [1, 2]
    }
}

2.2.3 判断
判断当前集合是否为空集合：

boolean isEmpty();

判断当前集合中是否存在一个与 obj 对象 equals 返回 true 的元素：

boolean contains(Object o);

判断 c 集合中的元素是否在当前集合中都存在，即 c 集合是否为当前集合的子集：

boolean containsAll(Collection<?> c)

示例：

package com.github.demo7;
import java.util.ArrayList;
import java.util.Collection;
/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class Test {
    public static void main(String[] args) {
        Collection<String> c1 = new ArrayList<>();
        c1.add("aa");
        c1.add("bb");
        c1.add("cc");
        c1.add("dd");
        System.out.println("c1 = " + c1.isEmpty()); // c1 = false
        c1.clear();
        System.out.println("c1 = " + c1.isEmpty()); // c1 = true
    }
}

示例：

package com.github.demo8;
import java.util.ArrayList;
import java.util.Collection;
/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class Test {
    public static void main(String[] args) {
        Collection<String> c1 = new ArrayList<>();
        c1.add("aa");
        c1.add("bb");
        c1.add("cc");
        c1.add("dd");
        System.out.println("c1 = " + c1.contains("aa")); // c1 = true
        System.out.println("c1 = " + c1.contains("aaa")); // c1 = false
    }
}

示例：

package com.github.demo9;
import java.util.ArrayList;
import java.util.Collection;
/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class Test {
    public static void main(String[] args) {
        Collection<String> c1 = new ArrayList<>();
        c1.add("aa");
        c1.add("bb");
        c1.add("cc");
        c1.add("dd");
        Collection<String> c2 = new ArrayList<>();
        c2.add("aa");
        c2.add("bb");
        c2.add("ee");
        System.out.println("c1.containsAll(c2) = " + c1.containsAll(c2)); // c1.containsAll(c2) = false
        Collection<String> c3 = new ArrayList<>();
        c2.add("aa");
        c2.add("bb");
        System.out.println("c1.containsAll(c3) = " + c1.containsAll(c3)); // c1.containsAll(c3) = true
    }
}

2.2.4 获取集合中元素的个数

• 获取当前集合中实际存储的元素个数：

int size();

示例：

package com.github.demo10;
import java.util.ArrayList;
import java.util.Collection;
/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class Test {
    public static void main(String[] args) {
        Collection<String> c1 = new ArrayList<>();
        c1.add("aa");
        c1.add("bb");
        c1.add("cc");
        c1.add("dd");
        System.out.println("c1.size() = " + c1.size()); // c1.size() = 4
        c1.clear();
        System.out.println("c1.size() = " + c1.size()); // c1.size() = 0
    }
}

2.2.5 交集

• 当前集合仅保留与 c 集合中的元素相同的元素，即当前集合中仅保留两个集合的交集：

boolean retainAll(Collection<?> c);

示例：

package com.github.demo11;
import java.util.ArrayList;
import java.util.Collection;
/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class Test {
    public static void main(String[] args) {
        Collection<String> c1 = new ArrayList<>();
        c1.add("aa");
        c1.add("bb");
        c1.add("cc");
        c1.add("dd");
        Collection<String> c2 = new ArrayList<>();
        c2.add("bb");
        c1.retainAll(c2);
        System.out.println("c1 = " + c1); // c1 = [bb]
    }
}

2.2.6 转数组
返回包含当前集合中所有元素的数组：

Object[] toArray();
<T> T[] toArray(T[] a);

示例：

package com.github.demo12;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class Test {
    public static void main(String[] args) {
        Collection<String> c = new ArrayList<>();
        c.add("aa");
        c.add("bb");
        c.add("cc");
        c.add("dd");
        Object[] objects = c.toArray();
        System.out.println("objects = " + Arrays.toString(objects)); // objects = [aa, bb, cc, dd]
        String[] strings = c.toArray(new String[c.size()]); 
        System.out.println("strings = " + Arrays.toString(strings)); // strings = [aa, bb, cc, dd]
    }
}