《JAVA编程思想(第四版)》第三四章(操作符/控制执行流程)小结

3. 操作符

对象内部称之为引用,(C中的指针),使用时要小心引用.

class Tank{
	int level;
}
public class Assignment {
	public static void main(String[] args) {
		Tank t1 = new Tank();
		Tank t2 = new Tank();
		t1.level = 9;
		t2.level = 47;
		System.out.println("1:t1.level:" + t1.level + ",t2.level :" + t2.level);
                //1:t1.level:9,t2.level :47
		t1 = t2;// 都指向了同一个对象
		System.out.println("2:t1.level:" + t1.level + ",t2.level :" + t2.level);
    		//2:t1.level:47,t2.level :47
                t1.level = 27; //两个引用指向的是同一块内存,所以都被改变
		System.out.println("3:t1.level:" + t1.level + ",t2.level :" + t2.level);
                 ////3:t1.level:27,t2.level :27
	}	
}

基本 类型转化.具体原理是由各个基本类型的位数决定的.

//: operators/Casting.java
public class Casting {
	public static void main(String[] args) {
		int i = 200;
		long lng = (long) i;
		lng = i; // 高类型得到低类型的值,不需要强转
		i = lng;	// 报错Type mismatch: cannot convert from long to int
		i = 'c';	//char 可以向上转为int,所以这语句正确,i = 99('c'的ASCII码值)
	}
        //出现这个问题的原因在于:
        //long 8个字节    -9223372036854775808 -> 9223372036854775807
        //int 4个字节    -2147483648 -> 2147483648
        //简单说就是long可以存下int,所以编译器不担心,但int有可能存不下long,编译器小心起见,需要程序员确认.
}

+=,-=,*=,/=,&=,|=,^= 这些语句都是合法的

如何得到各种基本类型的最大值和最小值?

public class Exponents {
	public static void main(String[] args) {
		double maxDouble = Double.MAX_VALUE;
		float maxFloat = Float.MAX_VALUE;
		int maxInt = Integer.MAX_VALUE;
		int minInt = Integer.MIN_VALUE;
//		short s = Short.MAX_VALUE;
//		byte b = Byte.MAX_VALUE;
//		long l = Long.MAX_VALUE;
//		char c = Character.MAX_VALUE;
			//以上都成立
		System.out.println("maxDouble = " + maxDouble +'\n' + "maxFloat = " + maxFloat);
		System.out.println("int的取值范围是:" + minInt + "-->" + maxInt);
	}
               /**
	         * maxDouble = 1.7976931348623157E308 maxFloat = 3.4028235E38
	         * int的取值范围是:-2147483648-->2147483647
	         */
}

切记 i=0; i = i++; //i依然==0

public class exp {
	public static void main(String[] args) {
		int i = 0;
		while(i<1) {
			i = i++;	//i = 0; (i++不起作用了)
			System.out.println(i);	//无限输出0
		}
	}
	
}

if(x = y) //在java 中一般会报错:编译时异常,除非x,y都是boolean 值

public class exp {
	public static void main(String[] args) {
		int x=0;
		int y=0;
		if(x=y) {//编译错误: Type mismatch: cannot convert from int to boolean
			
		}
		boolean x2 = false;
		boolean y2 = true;
		if(x2 = y2) {	//成功,并且表达式的值由y2,也就是右值决定.
			
		}
	}
}

4. 控制执行流程

这一章主要就是for,while,dowhile,有意思的大概也就java中的goto语句了.

java中不提供 goto 语句,但是保留 goto 关键字

break 和 continue 语句已经含有 goto 功能, 可以直接将goto 原有的lable 放在 break/continue 之后.

主要是用来控制多重循环.

public class LabeledFor {
	public static void main(String[] args) {
		int i = 0;
		outer: while (true) {
			inner: for (; i < 10; i++) {
				if (i == 2) {
					System.out.println("continue");
					continue;
				}
				if (i == 3) {
					System.out.println("break");
					i++;
					break;
				}
				if (i == 7) {
					System.out.println("continue outer");
					i++;
					continue outer;
				}
				if (i == 8) {
					System.out.println("break outer");
					break outer;
				}
				for (int k = 0; k < 5; k++) {
					if (k == 3) {
						System.out.println("continue inner");
						continue inner;
					}
				}
			}
		}
	}
}
/*
continue inner
continue inner
continue
break
continue inner
continue inner
continue inner
continue outer
break outer
*/