一、填空题
1.假设有
String s1 = "Welcome to Java";
String s2 = s1;
String s3 = new String("Welcome to Java");那么下面表达式的结果是什么？
(1) s1 == s2 ___________true_______________
(2) s1 == s3 ___________false______________
(3) s1.equals(s2) ___________true ______________
(4) s2.equals(s3) ___________true ______________
(5) s1.compareTo(s2); ____________0_________________
(6) s2.compareTo(s3); ____________0_________________
(7) s1.charAt(0); ____________’W’_______________
(8) s1.indexOf('j'); ____________-1________________
(9) s1.indexOf("to"); ____________8_________________
(10) s1.lastIndexOf("o",15) ___________9_________________
(11) s1.substring(3, 11); ________” come to ” ___________
(12) s1.endsWith("Java") ______________true________________
(13) s1.startsWith("wel"); _________false_________________
(14) " We come ".trim(); ________”We come”______________
(15) s1.toUpperCase(); ________”WELCOME TO JAVA”______
(16) s1.replace('o', 'T'); ________”WelcTme tT Java”______
2. 如果有
StringBuffer s1 = new StringBuffer("Java");
StringBuffer s2 = new StringBuffer("HTML");假设下列每个语句是独立的，每条语句结束后，写出相应结果：
(1) s1.append(" is fun"); s1为_____ Java is fun ___
(2) s1.append(s2); s1为_____ JavaHTML ______
(3) s1.insert(2, "is fun"); s1为______Jais funva_____
(4) s1.insert(1,s2); s1为______JHTMLava_______
(5) char c = s1.charAt(2); c为_______v______________
(6) int i = s1.length(); i为_______4_____________
(7) s1.deleteCharAt(3); s1为______Jav____________
(8) s1.delete(1,3); s1为______Ja______________
(9) s1.reverse(); s1为______avaJ____________
(10) s1.replace(1,3, "Computer"); s1为_____JComputera_______
(11) String s3 = s1.substring(1,3);
s3为____av_________________，s1为__________Java___________
(12) String s4 = s1.substring(2);
S4为____va______________，s1为__________Java_________________
3. 假设有
StringBuffer s = new StringBuffer("Welcome to JAVA");将s的内容清空的语句是_______ s.delete(0, s.length());________。
4. 如果有
String s1 = "Welcome";
String s2 = new String("Welcome");
String s3 = s2.intern();
String s4 = "Wel" + "come";
String s5 = "Wel";
String s6 = "come";
String s7 = s5 + s6;
String s8 = "Wel" + new String("come");那么下面表达式的结果为：
（1）s1 == s2 _____false_____
（2）s1 == s3 _____true______
（3）s1 == s4 _____true______
（4）s1 == s7 _____false_____
（5）s1 == s8 _____false_____
（6）s1.equals(s2) ____true________
（7）s1.equals(s3) ___true_________
（8）s1.equals(s4) ___true_________
（9）s1.equals(s7) ___true_________
（10）s1.equals(s8) ___true_________
二、单项选择题
1. 可以获取字符串s的最后一个字符的表达式是___C_____。
（A）s.length()
（B）s[s.length() - 1]
（C）s.charAt(s.length() - 1)
（D）charAt(s, length(s))
2. 下面程序
class C {public static void main(String[] args) {String s = “null”;if(s == null)System.out.print(“a”);else if(s.length() == 0)System.out.print(“b”);elseSystem.out.print(“c”);}
}的输出为___C_____。
（A）a （B）b
（C）c （D）null
3. 下面程序
class C {public static void main(String[] args) {String s = “Welcome to ”;concat(s);System.out.print(s);}public static void concat(String s) {s += “Java”;}
}的输出为___A___。
（A）Welcome to （B）Welcome to Java
（C）编译错误 （D）运行时异常
三、编程题
1. 编写程序，从控制台或对话框任意输入一个英文字符串，统计字符串中每个英文字母出现的次数并输出到控制台（大小写不敏感）。
import java.util.Scanner;
public class Main{public static void main(String[] args) {Scanner scanner = new Scanner(System.in);String s = scanner.next();s = s.toLowerCase();int [] cnt = new int [26];for(int i = 0; i < s.length(); i++){if(Character.isLetter(s.charAt(i))){cnt[s.charAt(i) - 'a']++;}}for(char i = 'a'; i <= 'z'; i++){System.out.print(i + ":"+cnt[i - 'a'] + " ");}}
}运行结果：
2.假设一个车牌号码由三个大写字母和后面的四个数字组成。编写一个程序. 生
成5个不重复的车牌号码。
public class Main{public static void main(String[] args) {String [] licensePlate = new String [5];int cnt = 0;while(cnt < 5){StringBuilder s = new StringBuilder();for (int i = 0; i < 3; i++) {char letter = (char) (26 * Math.random() + 'A');s.append(letter);}for(int i = 0; i < 4; i++){char num = (char)(10 * Math.random() + '0');s.append(num);}boolean flag = false;for(int i = 0; i < cnt; i++){if(licensePlate[i].contentEquals(s)){flag = true;break;}}if(!flag){licensePlate[cnt] = String.valueOf(s);cnt++;}}for(int i = 0; i < 5; i++){System.out.println(licensePlate[i]);}}
}运行结果：
