关于一些JAVA的基础知识总结

Java 基础

• 有如下两个数组 int a[] = {1,3,5,7,8}; int b[] = {0,3,5,10,20}; 选出属于数组b但不属于数组a的元素

public class TestInt {

public static void main(String[] args) {
int a[] = {1,3,5,7,8};
int b[] = {0,3,5,10,20};
        System.out.println(Arrays.toString(find(b,a)));
    }

//选出属于数组a但不属于数组b的元素
public static int[] find(int[] a, int[] b){
        ArrayList<Integer> list = new ArrayList<Integer>();
for(int i = 0; i < a.length; ++i) {
            boolean bContained = false;
for(int j = 0; j < b.length; ++j) {
if (a[i] == b[j]) {
                    bContained = true;
break;
                }
            }
if (!bContained) {
                list.add(a[i]);
            }
        }
int res[] = new int[list.size()];
for(int i = 0; i < list.size(); ++i){
            res[i] = list.get(i);
        }
return res;
    }
}

• 继承、静态的理解

public class TestStaticExtend {
public static void main(String[] args) {
        A se = new B();
        se.print();
    }
}

class A{
static {
        System.out.println("AAAA");
    }

public A(){
        System.out.println("struc A");
    }

    void print(){
        System.out.println("this is class A");
    }
}
class B extends A{
static{
        System.out.println("BBBB");
    }

public B(){
        System.out.println("struc B");
    }

    void print(){
        System.out.println("this is class B");
    }
}

结果:

AAAA
BBBB
struc A
struc B
this is class B

• String 的理解

public class TestString {
    String str = new String("hello world");
    String[] strarr = {"hello world"};
char[] ch = { 'a', 'b', 'c' };

static TestString ex = new TestString();
public static void main(String[] args) {

        change(ex.str, ex.ch);
        System.out.println(ex.getStr()+ " and " + Arrays.toString(ex.ch));

        change(ex.strarr, ex.ch);
        System.out.println(ex.strarr[0] + " and " + Arrays.toString(ex.ch));
    }

public static void change(String[] str, char[] ch) {    
        str[0] = "test ok";
        ch[0] = 'g';
    }

public static void change(String str, char[] ch) {
        ex.setStr("test ok");
        ch[0] = 'g';
    }

public String getStr() {
return str;
    }
public void setStr(String str) {
this.str = str;
    }
}

结果：

test ok and [g, b, c]
test ok and [g, b, c]