1单向if语句双向if语句dangling elseswitch:char,byte,short,int2javax.swing.JOptionPane.showConfirmDialog(null,text);返回值:JOptionPane.YES_OPTION:0JOptionPane.NO_OPTION:1JOptionPane.CANCEL_OPTION:23cannot cast int from booleancannot cast boolean from int4%b5printf("%f",2);java.util.IllegalFormatConversionException6java基本是值传参,基本类型不可改,需要Integer,Double打包,Field f = r1.getClass().getDeclaredField("value");f.setAccessible(true);f.setDouble(r1.r1+1);同时不能使用拷贝构造函数	public static boolean calEquRoot(double a,double b,double c,Double r1,Double r2) throws NoSuchFieldException, SecurityException, IllegalArgumentException, IllegalAccessException{		double delta = a * a - 4 * b * c;		if(delta < 0)return false;		delta=Math.sqrt(delta);		Field f=r1.getClass().getDeclaredField("value");		//r1=Double.valueof(1.0);r2=Double.valueof(2.0);		f.setAccessible(true);		f.setDouble(r1,Double.valueOf((-b+delta)/2/a));		f.setDouble(r2,Double.valueOf((-b-delta)/2/a));		return true;	}String类型为不可变字符串,传入也不改变其值Keyword:Boolean expressionBoolean valueboolean typebreak statementconditional operatordangling-else ambiguityfall-through behavior:语句从匹配处开始执行，直到遇到break语句或是达到switch语句的末端。operator associativityoperator precedenceselection statementshort-circuit evaluation3.1< <= >= != == >3.2cannot cast 3.330 is even30 is odd________________30 is even3.4z is 5________________z is 7________________x is 23.5a==c==d缩进正确:b,c3.6.............3.7等价3.80.5,0.0,0.2343.9Math.random()*20;Math.random()*10+10;Math.random()*40+10;3.10if(y > 0)x=1;3.11if(score > 90)pay+=1.03;if(score > 90)pay+=1.03;else pay*=1.01;3.12if (score >= 90.0)grade = ‘A‘;else if (score >= 80.0)grade = ‘B‘;else if (score >= 70.0)grade = ‘C‘;else if (score >= 60.0)grade = ‘D‘;else grade = ‘F‘;3.13boolean fl = count % 10 == 0;if (fl)newLine = true;else newLine = false;3.14false,false,true,true,true,true,3.15x >= 1 && x <= 1003.16(x >= 1 && x <= 100) || x<03.17x = y && yx /= y(x != 0) || (x = 0)3.18223.19true,false,true,false3.20(x < y && y < z) is true(x < y || y < z) is true!(x < y) is false(x + y < z) is false(x + y < z) is false3.21age > 13 && age < 183.22weight > 50 || height > 1603.23weight > 50 && height > 1603.24weight > 50 ^ height > 1603.25char,byte,int,long下一个标签内的语句可以可能不行格式紧凑,可读性好3.2623.27switch(a){case 1:	x+=5;	break;case 2:	x+=10;	break;case 3:	x+=16;	break;case 4:	x+=34;}3.28switch(day){case 0:	dayName = "Sunday";	break;case 1:	dayName = "Monday";	break;case 2:	dayName = "Tuesday";	break;case 3:	dayName = "Wednesday";	break;case 4:	dayName = "Thursday";	break;case 5:	dayName = "Friday";	break;case 6:	dayName = "Saturday";	break;}3.29System.out.print(count + count % 10 ==0?"\n":" ");3.30pay*=(temperature > 90?1.5:1.1);3.31%b %c %d %f %s3.32变参个数多变参个数少java.util.IllegalFormatConversionException 3.33amount is 32.32 3.232000e+01amount is 32.3200 3.2320e+01false Java   falseJavafalse Java   3.35truetrue3.36假,赋值运算符(简捷运算符/=...)3.37falsefalse3.38yes,yes,yes3.39javax.swing.JOptionPane.showConfirmDialog(null,confirmMessage);JOptionPane.YES_OPTION:0JOptionPane.NO_OPTION:1JOptionPane.CANCEL_OPTION:23.1import java.lang.reflect.Field;import java.util.Scanner;import java.text.*;public class Exercise {	public static int calEquRoot(double a,double b,double c,Double r1,Double r2) throws NoSuchFieldException, SecurityException, IllegalArgumentException, IllegalAccessException{				double delta = b * b - 4 * a * c;		if(delta < 0)return -1;		delta=Math.sqrt(delta);		Field f=r1.getClass().getDeclaredField("value");		f.setAccessible(true);		f.setDouble(r1,Double.valueOf((-b+delta)/2/a));		f.setDouble(r2,Double.valueOf((-b-delta)/2/a));		if(delta < 1e-10)return 0;		return 1;	}	public static void main(String[] args) throws NoSuchFieldException, SecurityException, IllegalArgumentException, IllegalAccessException{		double a,b,c;		Double r1 = Double.valueOf(0),r2 = Double.valueOf(0);		Scanner scanner = new Scanner(System.in);		System.out.println("Input a:");		a = scanner.nextDouble();		System.out.println("Input b:");		b = scanner.nextDouble();		System.out.println("Input c:");		c = scanner.nextDouble();		int fl=calEquRoot(a,b,c,r1,r2);		if(fl == 1){			System.out.println(r1);			System.out.println(r2);		}		else if(fl == 0){			System.out.println(r1);		}		else {			System.out.println("NO ROOT");		}	}}3.9import java.util.Scanner;public class Exercise {	public static void main(String[] args) throws Exception{		Scanner scanner = new Scanner(System.in);		int[] a = new int[9];		int ans=0;		String tmp=scanner.next();		for(int i=0;i<9;i++){			a[i]=tmp.charAt(i)-‘0‘;			ans+=(a[i] * (i + 1)) % 11;		}		ans%=11;		System.out.println(tmp+ans);	}}3.17import java.util.Scanner;public class Exercise {	public static void main(String[] args) throws Exception{		@SuppressWarnings("resource")		Scanner scanner = new Scanner(System.in);		int r = (int)Math.random() * 3;		String[] srp={"scissor","rock","paper"};		String[] result={"You won","You lose","It is a draw"};		System.out.print("scissor(0), rock(1), paper(2): ");		int a;		while((a=scanner.nextInt())<=2 && a>=0){			System.out.print("The computer is "+srp[r]+". You are "+srp[a]+(a==r?" too. ":". ")					+ (a==r?result[2]:(a-r==1||a-r==-2)?result[0]:result[1]));		}	}}3.20import java.util.Scanner;public class Exercise {	public static void main(String[] args) throws Exception{		@SuppressWarnings("resource")		Scanner scanner = new Scanner(System.in);		System.out.print("Enter the temperature in Fahrenheit: ");		double ta;		while((ta=scanner.nextDouble()) < -58||ta > 41){			System.out.print("Enter the temperature in Fahrenheit bewteen -58 and 41: ");		}		double v;		System.out.print("Enter the wind speed miles per hour: ");		while((v=scanner.nextDouble()) < 2){			System.out.print("Enter the wind speed miles per hour above 2 miles/h: ");		}		double ans=35.74 + 0.6215 * ta - 35.75 * Math.pow(v, 0.16) + 0.4275 * ta * Math.pow(v, 0.16);		System.out.println("The wind chill index is "+ans);	}}3.21import java.util.Scanner;public class Exercise {	/**	 * 泽勒一致性	 * */	public static void main(String[] args) throws Exception{		@SuppressWarnings("resource")		Scanner scanner = new Scanner(System.in);		int year,month,day,week;		System.out.print("Enter year: ");		year = scanner.nextInt();		System.out.print("Enter month: ");		month = scanner.nextInt();		if(month < 3)month+=12;		System.out.print("Enter the day of the month: ");		day = scanner.nextInt();		String[] date = {				"Saturday","Sunday",				"Monday","Tuesday","Wednesday",				"Thursday","Friday"		};		week = (day + 26*(month + 1)/10 + year % 100 + year % 100 / 4 + year/400 + year/100*5)%7;		System.out.println("Day of the week is " + date[week]);	}}3.22import java.util.Scanner;public class Exercise {	public static void main(String[] args) throws Exception{		@SuppressWarnings("resource")		Scanner scanner = new Scanner(System.in);		double x,y;		System.out.print("Enter a point with two coordinates: ");		x = scanner.nextDouble();		y = scanner.nextDouble();		if(x * x + y * y <= 100){			System.out.println("Point ("+x+", "+y+") is in the circle");		}		else {			System.out.println("Point ("+x+", "+y+") is not in the circle");		}	}}3.27import java.util.Scanner;public class Exercise {	final static double eps=1e-16;	public static boolean judge(double a,double b,double x,double y){		if(Math.abs(a - b) < eps && a < eps ){			if(Math.abs(x - y) < eps && x < eps){				return true;			}			else return false;		}		else{			boolean fl = a * b < 0;			boolean fl2 = (b * x + a * y - a * b) > 0;			if(fl^fl2){				return false;			}			else return true;		}	}	public static void main(String[] args) throws Exception{		@SuppressWarnings("resource")		Scanner scanner = new Scanner(System.in);		double x,y;		System.out.print("Enter a point with two coordinates: ");		x = scanner.nextDouble();		y = scanner.nextDouble();		if(judge(200,100,x,y)){			System.out.println("Point ("+x+", "+y+") is in the circle");		}		else {			System.out.println("Point ("+x+", "+y+") is not in the circle");		}	}}3.28import java.util.Scanner;public class Exercise {	final static double eps=1e-16;	public static int judge(double[] w,double[] h,double[] x,double[] y){		w[0]/=2;w[1]/=2;h[0]/=2;h[1]/=2;		double dx = Math.abs(x[0] - x[1]);				double dy = Math.abs(y[0] - y[1]);		if(dx > w[0] + w[1]||dy > h[0] + h[1])return 0;		if(dx > w[0]||dy > h[0])return 2;		return 1;	}	public static void main(String[] args) throws Exception{		@SuppressWarnings("resource")		Scanner scanner = new Scanner(System.in);		double[] x = new double [2],y = new double [2],w = new double [2],h = new double [2];		System.out.println("Enter r1‘s center x-, y-coordinates, width, and height: ");		x[0] = scanner.nextDouble();		y[0] = scanner.nextDouble();				w[0] = scanner.nextDouble();		h[0] = scanner.nextDouble();		System.out.println("Enter r2‘s center x-, y-coordinates, width, and height: ");		x[1] = scanner.nextDouble();		y[1] = scanner.nextDouble();				w[1] = scanner.nextDouble();		h[1] = scanner.nextDouble();		int fl;		if((fl = judge(w,h,x,y)) == 0){			System.out.println("r2 does not overlap r1");		}		else if(fl == 1){			System.out.println("r2 is inside r1");		}		else {			System.out.println("r2 overlaps r1");		}	}}