我最近透過[https://exercism.org/tracks/java/exercises]中的實作學習了Java。我目前的進度是 148 次練習中的 13 次。我想分享我所學到的東西。
這篇文章介紹了我對 .split()、.trim()、.isDigit()、.isLetter()、Comparable
定義：.split()方法依分隔符號[1]將String分割成陣列。
文法:
public String[] split(String regex, int limit)
參數：
範例：
public class Main{
public void getProductPrice(String products){
double totalPrice = 0.0;
StringBuilder priceDetails = new StringBuilder();
String[] singleProduct = products.split("; ");
for(int i = 0; i < singleProduct.length; i++){
String[] productInfo = singleProduct[i].split(", ");
totalPrice += Double.parseDouble(productInfo[2]);
priceDetails.append(productInfo[2]);
if(i < singleProduct.length - 1){
priceDetails.append(" + ");
}
}
System.out.println(priceDetails + " = " + totalPrice);
}
public static void main(String arg[]){
Main obj = new Main();
obj.getProductPrice("1, dragonfruit, 12.50; 2, guava, 23.45; 3, avocado, 395.67");
}
}
輸出:
12.50 + 23.45 + 395.67 = 431.62
定義：.trim() 方法刪除字串兩端的空格 [2]。
文法:
public String trim()
參數：
範例：
public class Main{
public static void main(String args[]){
String str = " You can do it! ";
System.out.println(str);
System.out.println(str.trim());
}
}
輸出:
You can do it! You can do it!
定義：.isDigit() 方法判斷字元是否為數字 [3].
文法:
public static boolean isDigit(char ch)
參數：
範例：
public class Main{
// return true when the given parameter has a digit
public boolean searchDigit(String str){
for(int i = 0; i < str.length(); i++){
// charAt() method returns the character at the specified index in a string
if(Character.isDigit(str.charAt(i))){
return true;
}
}
return false;
}
// print digit index and value
public void digitInfo(String str){
for(int i = 0; i < str.length(); i++){
if(Character.isDigit(str.charAt(i))){
System.out.println("Digit: " + str.charAt(i) + " found at index " + i);
}
}
}
public static void main(String args[]){
Main obj = new Main();
String[] strList = {"RT7J", "1EOW", "WBJK"};
for(String str : strList){
if(obj.searchDigit(str)){
obj.digitInfo(str);
}else{
System.out.println("No digit");
}
}
}
}
輸出:
Digit: 7 found at index 2 Digit: 1 found at index 0 No digit
定義：.isLetter() 方法決定字元是否為字母 [4].
文法:
public static boolean isLetter(char ch)
參數：
範例：
public class Main{
// check whether phoneNum has letter
public void searchLetter(String phoneNum){
boolean hasLetter = false;
for(int i = 0; i < phoneNum.length(); i++){
if(Character.isLetter(phoneNum.charAt(i))){
hasLetter = true;
// return letter value and index
System.out.println(phoneNum + " has letter '" + phoneNum.charAt(i) + "' at index " + i);
}
}
// phone number is valid when no letter
if(!hasLetter){
System.out.println(phoneNum + " is valid");
}
System.out.println();
}
public static void main(String args[]){
Main obj = new Main();
String[] phoneNum = {"A0178967547", "0126H54786K5", "0165643484"};
for(String item: phoneNum){
obj.searchLetter(item);
}
}
}
輸出:
A0178967547 has letter 'A' at index 0 0126H54786K5 has letter 'H' at index 4 0126H54786K5 has letter 'K' at index 10 0165643484 is valid
定義：可比較的
範例：
// file: Employee.java
public class Employee implements Comparable<Employee>{
private String email;
private String name;
private int age;
public Employee(String email, String name, int age){
this.email = email;
this.name = name;
this.age = age;
}
// The Comparable interface has a method called compareTo(T obj).
// This method helps decide how to order objects, so they can be sorted in a list
@Override
public int compareTo(Employee emp){
// compare age:
// return this.age - emp.age;
// (this.age - emp.age) = negative value means this.age before emp.age;
// (this.age - emp.age) = positive means this.age after emp.age
// compare email:
return this.email.compareTo(emp.email);
}
@Override
public String toString(){
return "[email=" + this.email + ", name=" + this.name + ", age=" + this.age +"]";
}
}
// file: Main.java
import java.util.Arrays;
public class Main {
public static void main(String args[]){
Employee[] empInfo = new Employee[3];
empInfo[0] = new Employee("joseph@gmail.com", "Joseph", 27);
empInfo[1] = new Employee("alicia@gmail.com", "Alicia", 30);
empInfo[2] = new Employee("john@gmail.com", "John", 24);
Arrays.sort(empInfo);
System.out.println("After sorting:\n" + Arrays.toString(empInfo));
}
}
輸出:
After sorting: [[email=alicia@gmail.com, name=Alicia, age=30], [email=john@gmail.com, name=John, age=24], [email=joseph@gmail.com, name=Joseph, age=27]]
定義：使用者定義的 Java 異常是開發人員為處理特定錯誤條件而建立的自訂異常 [6]。
範例：
public String[] split(String regex, int limit)
public class Main{
public void getProductPrice(String products){
double totalPrice = 0.0;
StringBuilder priceDetails = new StringBuilder();
String[] singleProduct = products.split("; ");
for(int i = 0; i < singleProduct.length; i++){
String[] productInfo = singleProduct[i].split(", ");
totalPrice += Double.parseDouble(productInfo[2]);
priceDetails.append(productInfo[2]);
if(i < singleProduct.length - 1){
priceDetails.append(" + ");
}
}
System.out.println(priceDetails + " = " + totalPrice);
}
public static void main(String arg[]){
Main obj = new Main();
obj.getProductPrice("1, dragonfruit, 12.50; 2, guava, 23.45; 3, avocado, 395.67");
}
}
輸出:
12.50 + 23.45 + 395.67 = 431.62
Java 中的介面允許使用者跨不同類別呼叫相同的方法，每個類別都實作自己的邏輯 [7]。在下面的範例中，方法calculatePrice()在不同的類別中調用，例如Fruit和DiscountFruit，每個類別應用自己獨特的計算邏輯。
範例：
public String trim()
public class Main{
public static void main(String args[]){
String str = " You can do it! ";
System.out.println(str);
System.out.println(str.trim());
}
}
You can do it! You can do it!
public static boolean isDigit(char ch)
輸出:
public class Main{
// return true when the given parameter has a digit
public boolean searchDigit(String str){
for(int i = 0; i < str.length(); i++){
// charAt() method returns the character at the specified index in a string
if(Character.isDigit(str.charAt(i))){
return true;
}
}
return false;
}
// print digit index and value
public void digitInfo(String str){
for(int i = 0; i < str.length(); i++){
if(Character.isDigit(str.charAt(i))){
System.out.println("Digit: " + str.charAt(i) + " found at index " + i);
}
}
}
public static void main(String args[]){
Main obj = new Main();
String[] strList = {"RT7J", "1EOW", "WBJK"};
for(String str : strList){
if(obj.searchDigit(str)){
obj.digitInfo(str);
}else{
System.out.println("No digit");
}
}
}
}
[1] JavaRush，java 中的 split 方法：將字串拆分為多個部分，2023 年 8 月 8 日
[2] W3Schools，Java String trim() 方法
[3] GeeksforGeeks，Java 中的 Character isDigit() 方法及其範例，2020 年 5 月 17 日
[4] 教學點，Java - 字元 isLetter() 方法
[5] Java 範例中的 DigitalOcean、Comparable 和 Comparator，2022 年 8 月 4 日
[6] Shiksha，了解 Java 中的使用者定義異常，2024 年 4 月 25 日
[7] Scientech Easy，Java 中的介面使用範例，2024 年 7 月 9 日
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