3、变量

变量

1、为什么需要变量

image-20210708114846518

1.1、变量是程序的基本组成单位

image-20210708115206159

1.2、简单的原理示意图

image-20210708133958062

2、变量的介绍

image-20210708134044608

3、变量快速入门

image-20210708134643817

public class Var01 {
	public static void main(String[] args) {
		//定义变量
		String name = "king";	//姓名
		int age = 22;			//年龄
		char gender = '男';		//性别
		double score = 88.8;	//成绩

		System.out.println("人的信息如下：");
		System.out.println(name);
		System.out.println(age);
		System.out.println(gender);
		System.out.println(score);
	}
}

image-20210708135642794

4、变量使用注意事项

image-20210708135743825

5、程序中 + 号的使用

image-20210708141248067

public class Plus {
	public static void main(String[] args) {
		System.out.println(100 + 98);
		System.out.println("100" + 98);
		System.out.println(100 + 3 + "hello");
		System.out.println("hello" + 100 + 3);
	}
}

image-20210708141743242

6、数据类型

image-20210708142638267

6.1、整数类型

image-20210708142827385

image-20210708143212831

image-20210708143618333

6.2、浮点类型

image-20210708151601367

image-20210708151818685

image-20210708164630882

public class FloatDetail {
	public static void main(String[] args) {
		//十进制数形式
		double num1 = 5.12;
		float num2 = 512.0F;
		double num3 = .512;
		System.out.println(num1);
		System.out.println(num2);
		System.out.println(num3);
		//科学计数法形式
		double num4 = 5.12e2;
		double num5 = 5.12E-2;
		System.out.println(num4);
		System.out.println(num5);
		//double型比float型更精确
		double num6 = 2.1234567851;
		float num7 = 2.1234567851F;
		System.out.println(num6);
		System.out.println(num7);
		//浮点数使用陷阱
		double num8 = 2.7;
		double num9 = 8.1 / 3;
		System.out.println(num8);
		System.out.println(num9); //接近2.7的一个小数，而不是2.7
		//重要的使用点：当我们对运算结果是小数的进行判断时，要小心
		//以两个数的差值的绝对值，在某个精度范围类判断
		if ( num8 == num9 ) {
			System.out.println("相等");
		}
		//正确的写法
		System.out.println(Math.abs(num8 - num9));
		if ( Math.abs(num8 - num9) < 0.0001 ) {
			System.out.println("差值非常小，到了规定精度，认为相等...");
		}
	}
}

image-20210708170118481

6.3、字符类型

6.3.1、基本介绍

image-20210708174642771

public class Char01 {
	public static void main(String[] args) {
		char c1 = 'a';
		char c2 = '\t';
		char c3 = '多';
		char c4 = 97;

		System.out.println(c1);
		System.out.println(c2);
		System.out.println(c3);
		System.out.println(c4);
	}
}

image-20210708174955183

6.3.2、字符类型使用细节

image-20210708223406011

public class CharDetail {
	public static void main(String[] args) {

		//在java中，char的本质是一个整数，在默认输出时，是unicode码对应的字符
		//要输出对应的数字，可以(int)字符
		char c1 = 97;
		System.out.println(c1);
		char c2 = 'a';
		System.out.println((int)c2);
		char c3 = 38472;
		System.out.println(c3);
		char c4 = '陈';
		System.out.println((int)c4);

		//char类型是可以进行运算的，相当于一个整数，因为它都对应有Unicode码
		char c5 = 'a' + 10;
		System.out.println(c5);
		System.out.println((int)c5);
	}
}

image-20210708225004901

image-20210708230132860

image-20210708230147340

6.4、布尔类型：boolean

6.4.1、基本类型

image-20210708231401838

public class Boolean01 {
	public static void main(String[] args) {
		//判断成绩是否通过
		boolean flag = true;
		if ( flag ) {
			System.out.println("通过考试");
		} else {
			System.out.println("没有通过考试");
		}
	}
}

image-20210708231927477

6.4.2、使用细节说明

image-20210708232016226

7、Java API文档

image-20210708170224385

image-20210708171315139

8、ASCII 码介绍

image-20210708230909975

9、Unicode编码介绍

image-20210708230949390

10、UTF-8编码介绍

image-20210708231158365

11、基本数据类型转换

11.1、自动类型转换

image-20210708232106785

public class AutoConvert {
	public static void main(String[] args) {
		//自动转换
		int a = 'c';
		System.out.println(a);
		double d = 88;
		System.out.println(d);
	}
}

image-20210708232656770

11.2、自动类型转换注意和细节

image-20210708234134108

public class AutoConvertDetail {
	public static void main(String[] args) {
		int n1 = 10;
		double d1 = n1 + 1.1;
		float f1 = n1 + 1.1F;
		//强制转换
	    float f2 = n1 + (float)1.1;
		System.out.println(d1);
		System.out.println(f1);
		System.out.println(f2);
	}
}

image-20210708234501866

image-20210708235329106

public class AutoConvertDetail02 {	public static void main(String[] args) {		byte b1 = 1;		short s1 = 52;		char c1 = 'a';		int sum = b1 + s1 + c1;		System.out.println(sum);		double d1 = 66.6;		double sum2 = b1 + s1 + c1 + d1;		System.out.println(sum2);	}}

image-20210708235911790

11.3、强制类型转换

image-20210709094103121

public class ForceConvert {	public static void main(String[] args) {		//强制类型转换		int n = (int)1.6;		System.out.println("n = " + n);		//造成精度损失		int i = 1999;		byte b = (byte)i;		System.out.println("i = " + i);		System.out.println("b = " + b);		//造成数据溢出	}}

image-20210709094452162

11.4、强制类型转换细节说明

image-20210709100921685

public class ForceConvertDetail {	public static void main(String[] args) {		//int x = (int)10 * 3.5 + 6 * 1.5; //错误		int x = (int)(10 * 3.5 + 6 * 1.5);		System.out.println(x);		char c1 = 100;		int m = 100;		//char c2 = m; //错误		char c3 = (char)m;		System.out.println(c1);		System.out.println(m);		System.out.println(c3);	}}

image-20210709101320961

11.5、练习题

image-20210709102014502

public class ConvertTest {	public static void main(String[] args) {		short s = 12;		System.out.println(s);		//s = s - 9;	//错误，int --> short		System.out.println(s);		//正确写法		s = (short)(s - 9);		System.out.println(s);		byte b = 10;		//b = b + 11;	//错误，int --> byte		//正确写法		b = (byte)(b + 11);		char c = 'a';		int i = 16;		float d = .314F;		double result = c + i + d;		System.out.println(result);		byte b1 = 16;		short s1 = 14;		//short t = s1 + b1;	//错误，int --> short		//正确写法		short t = (short)(s1 + b1);		System.out.println(t);	}}

image-20210709104156490

12、基本数据类型和String类型的转换

image-20210709104302111

public class StringToBasic {	public static void main(String[] args) {		//基本数据类型 --> String		int n1 = 100;		float f1 = 1.1F;		double d1 = 3.14;		boolean b1 = true;		String s1 = n1 + "";		String s2 = f1 + "";		String s3 = d1 + "";		String s4 = b1 + "";		System.out.println(s1);		System.out.println(s2);		System.out.println(s3);		System.out.println(s4);		System.out.println("================");		//String --> 对应的基本数据类型		String s5 = "123";		byte num1 = Byte.parseByte(s5);		short num2 = Short.parseShort(s5);		int num3 = Integer.parseInt(s5);		long num4 = Long.parseLong(s5);		float num5 = Float.parseFloat(s5);		double num6 = Double.parseDouble(s5);		boolean b = Boolean.parseBoolean("true");		System.out.println(num1);		System.out.println(num2);		System.out.println(num3);		System.out.println(num4);		System.out.println(num5);		System.out.println(num6);		System.out.println(b);		//把字符串转成字符char --> 含义是指 把字符串的第一个字符得到 		char c1 = s5.charAt(0);		char c2 = s5.charAt(1);		char c3 = s5.charAt(2);		//char c4 = s5.charAt(3);		System.out.println(c1);		System.out.println(c2);		System.out.println(c3);		//System.out.println(c4);	//下标越界	}}

image-20210709111006516

12.1、注意事项

image-20210709111051445

public class StringToBasicDetail {	public static void main(String[] args) {		//字符串转基本数据类型的细节		String s1 = "123";		int n1 = Integer.parseInt(s1);		System.out.println(n1);		String s2 = "hello";		int n2 = Integer.parseInt(s2);		System.out.println(n2);	}}

image-20210709111925870

13、作业

image-20210709112030393

public class Homework01 {	public static void main(String[] args) {		int n1;		n1 = 13;		int n2;		n2 = 17;		int n3;		n3 = n1 + n2;		System.out.println("n3 = " + n3);	//30		int n4 = 38;		int n5 = n4 - n3;		System.out.println("n5 = " + n5);	//8	}}

image-20210709112354588

image-20210709112529687

2：

public class Homework02 {	public static void main(String[] args) {		char c1 = '\n';		//换行		char c2 = '\t';		//制表位		char c3 = '\r';		//回车		char c4 = '\\';		//输出\		char c5 = '1';		char c6 = '2';		char c7 = '3';		System.out.println(c1);		System.out.println(c2);		System.out.println(c3);		System.out.println(c4);		System.out.println(c5);		System.out.println(c6);		System.out.println(c7);	}}

image-20210709112856315

3：

public class Homework03 {	public static void main(String[] args) {		String book1 = "三国演义";		String book2 = "水浒传";		String s1 = book1 + book2;		System.out.println(book1);		System.out.println(book2);		System.out.println(s1);		char c1 = '男';		char c2 = '女';		//char c3 = c1 + c2;	//错误  int --> char		int i1 = c1 + c2;	//男字符码值 + 女字符码值		System.out.println(c1);		System.out.println(c2);		System.out.println(i1);		double price1 = 22.26;		double price2 = 66.68;		double price3 = price1 + price2;		System.out.println(price1);		System.out.println(price2);		System.out.println(price3);	}}

4：

public class Homework04 {	public static void main(String[] args) {		String name = "多多";	//姓名		int age = 2;			//年龄		double score = 66.6;	//成绩		char gender = '女';		//性别		String hobby = "吃";		//爱好		System.out.println("姓名\t年龄\t成绩\t性别\t爱好\n" + name + "\t" + age + "\t" + score + "\t" + gender + "\t" + hobby);	}}

image-20210709115500608

posted @ 2021-08-02 23:46  多瑞C  阅读(98)  评论(0)    收藏  举报