Java 學習記錄65 — Maps 1/3

今天要學的是 Map

import java.util.HashMap;
import java.util.Map;
public class MapProgram {
    public static void main(String[] args){
        Map<String, String> languages = new HashMap<>();
        languages.put("Java", " a compiled high level, object-oriented, platform independent language.");
        languages.put("Python"," an interpreted, object-oriented, high-level programming language.");
        languages.put("Algol"," an algorithmic language");
        languages.put("BASIC"," Beginners All Purposes Symbolic Instruction Code.");
        languages.put("Lisp"," Therein lies madness");
        System.out.println(languages.get("Java"));
    }
}

輸出結果：

a compiled high level, object-oriented, platform independent language.

map 是一組 key 跟 value

選定一個就可以得出另一個

補充：

為什麼要寫作 Map<String, String> languages = new HashMap<>()

前面跟後面的 type 不一樣呢？

因為 Map 本身是 Interface 是不能 create object

所以透過 HashMap 來實現

繼承圖

import java.util.HashMap;
import java.util.Map;
public class MapProgram {
    public static void main(String[] args){
        Map<String, String> languages = new HashMap<>();
        languages.put("Java", " a compiled high level, object-oriented, platform independent language.");
        languages.put("Python"," an interpreted, object-oriented, high-level programming language.");
        languages.put("Algol"," an algorithmic language");
        languages.put("BASIC"," Beginners All Purposes Symbolic Instruction Code.");
        languages.put("Lisp"," Therein lies madness");
        System.out.println(languages.get("Java"));
        languages.put("Java", " this course is about Java");
        System.out.println(languages.get("Java"));
    }
}

輸出結果：

a compiled high level, object-oriented, platform independent language.

this course is about Java

重寫就覆蓋掉了

import java.util.HashMap;
import java.util.Map;
public class MapProgram {
    public static void main(String[] args){
        Map<String, String> languages = new HashMap<>();
        languages.put("Java", " a compiled high level, object-oriented, platform independent language.");
        languages.put("Python"," an interpreted, object-oriented, high-level programming language.");
        languages.put("Algol"," an algorithmic language");
        languages.put("BASIC"," Beginners All Purposes Symbolic Instruction Code.");
        languages.put("Lisp"," Therein lies madness");
//        System.out.println(languages.get("Java"));
        System.out.println(languages.put("Java", " this course is about Java"));
        System.out.println(languages.get("Java"));
    }
}

輸出結果：

a compiled high level, object-oriented, platform independent language.

this course is about Java

用這個方法可得知上一個設定的值為何

倒數第二行打印的就是上一個值

import java.util.HashMap;
import java.util.Map;
public class MapProgram {
    public static void main(String[] args){
        Map<String, String> languages = new HashMap<>();
//        languages.put("Java", " a compiled high level, object-oriented, platform independent language.");
//        languages.put("Python"," an interpreted, object-oriented, high-level programming language.");
//        languages.put("Algol"," an algorithmic language");
        System.out.println(languages.put("BASIC", " Beginners All Purposes Symbolic Instruction Code."));
        System.out.println(languages.put("Lisp", " Therein lies madness"));
//        System.out.println(languages.get("Java"));
//        System.out.println(languages.put("Java", " this course is about Java"));
//        System.out.println(languages.get("Java"));
    }
}

輸出結果：

null

null

此為證明，上一個值尚未設定

import java.util.HashMap;
import java.util.Map;
public class MapProgram {
    public static void main(String[] args) {
        Map<String, String> languages = new HashMap<>();
        if (languages.containsKey("Java")) {
            System.out.println("Java already exists");
        } else {
            languages.put("Java", " a compiled high level, object-oriented, platform independent language.");
            System.out.println("Java added successfully");
        }
        languages.put("Python", " an interpreted, object-oriented, high-level programming language.");
        languages.put("Algol", " an algorithmic language");
        languages.put("BASIC", " Beginners All Purposes Symbolic Instruction Code.");
        languages.put("Lisp", " Therein lies madness");
        if (languages.containsKey("Java")) {
            System.out.println("Java is already in the map.");
        } else {
            System.out.println(languages.put("Java", " this course is about Java"));
        }
    }
}

輸出結果：

Java added successfully

Java is already in the map.

這樣就可以確保若已經有相同的 key，而 value 不會被覆寫

import java.util.HashMap;
import java.util.Map;
public class MapProgram {
    public static void main(String[] args) {
        Map<String, String> languages = new HashMap<>();
        languages.put("Java", " a compiled high level, object-oriented, platform independent language.");
        languages.put("Python", " an interpreted, object-oriented, high-level programming language.");
        languages.put("Algol", " an algorithmic language");
        languages.put("BASIC", " Beginners All Purposes Symbolic Instruction Code.");
        languages.put("Lisp", " Therein lies madness");

        for (String key : languages.keySet()) {
            System.out.println(key + " : " + languages.get(key));
        }
    }
}

輸出結果：

Java : a compiled high level, object-oriented, platform independent language.

BASIC : Beginners All Purposes Symbolic Instruction Code.

Lisp : Therein lies madness

Python : an interpreted, object-oriented, high-level programming language.

Algol : an algorithmic language

循環打印 ，但注意打印後的順序跟前面放入時不同

import java.util.HashMap;
import java.util.Map;
public class MapProgram {
    public static void main(String[] args) {
        Map<String, String> languages = new HashMap<>();
        languages.put("Java", " a compiled high level, object-oriented, platform independent language.");
        languages.put("Python", " an interpreted, object-oriented, high-level programming language.");
        languages.put("Algol", " an algorithmic language");
        languages.put("BASIC", " Beginners All Purposes Symbolic Instruction Code.");
        languages.put("Lisp", " Therein lies madness");
        languages.remove("Lisp");
        if(languages.remove("Algol", "a family of algorithmic languages")){
            System.out.println("Algol removed");
        }else{
            System.out.println("Algo not removed.");
        }

        for (String key : languages.keySet()) {
            System.out.println(key + " : " + languages.get(key));
        }
    }
}

輸出結果：

Algo not removed.

Java : a compiled high level, object-oriented, platform independent language.

BASIC : Beginners All Purposes Symbolic Instruction Code.

Python : an interpreted, object-oriented, high-level programming language.

Algol : an algorithmic language

put 是新增，移除就是 remove

移除 lisp

並檢測有無移除 Algol 且 value 也要對應，相同才會移除

此處 value 並不相同，所以沒被移除

import java.util.HashMap;
import java.util.Map;
public class MapProgram {
    public static void main(String[] args) {
        Map<String, String> languages = new HashMap<>();
        languages.put("Java", " a compiled high level, object-oriented, platform independent language.");
        languages.put("Python", " an interpreted, object-oriented, high-level programming language.");
        languages.put("Algol", " an algorithmic language");
        languages.put("BASIC", " Beginners All Purposes Symbolic Instruction Code.");
        languages.put("Lisp", " Therein lies madness");
        languages.remove("Lisp");
        if(languages.remove("Algol", " an algorithmic language")){
            System.out.println("Algol removed");
        }else{
            System.out.println("Algo not removed.");
        }

        for (String key : languages.keySet()) {
            System.out.println(key + " : " + languages.get(key));
        }
    }
}

輸出結果：

Algol removed

Java : a compiled high level, object-oriented, platform independent language.

BASIC : Beginners All Purposes Symbolic Instruction Code.

Python : an interpreted, object-oriented, high-level programming language.

這次的 key Algol 跟 value 都對應到了，所以被移除了

import java.util.HashMap;
import java.util.Map;
public class MapProgram {
    public static void main(String[] args) {
        Map<String, String> languages = new HashMap<>();
        languages.put("Java", " a compiled high level, object-oriented, platform independent language.");
        languages.put("Python", " an interpreted, object-oriented, high-level programming language.");
        languages.put("Algol", " an algorithmic language");
        languages.put("BASIC", " Beginners All Purposes Symbolic Instruction Code.");
        languages.put("Lisp", " Therein lies madness");
        System.out.println(languages.replace("Lisp", "a functional programming language with imperative features."));
        System.out.println(languages.replace("Scala", "this will not be added."));
        for (String key : languages.keySet()) {
            System.out.println(key + " : " + languages.get(key));
        }
    }
}

輸出結果：

Therein lies madness

null

Java : a compiled high level, object-oriented, platform independent language.

BASIC : Beginners All Purposes Symbolic Instruction Code.

Lisp : a functional programming language with imperative features.

Python : an interpreted, object-oriented, high-level programming language.

Algol : an algorithmic language

先打印了原本 Lisp 的 value

scala 本來就沒設定所以是 null

後來更新了 Lisp 先前的值

import java.util.HashMap;
import java.util.Map;
public class MapProgram {
    public static void main(String[] args) {
        Map<String, String> languages = new HashMap<>();
        languages.put("Java", " a compiled high level, object-oriented, platform independent language.");
        languages.put("Python", " an interpreted, object-oriented, high-level programming language.");
        languages.put("Algol", " an algorithmic language");
        languages.put("BASIC", " Beginners All Purposes Symbolic Instruction Code.");
        languages.put("Lisp", " Therein lies madness");
        if (languages.replace("Lisp", "This will not work", "a functional programming language with imperative features.")) {
            System.out.println("List replace");
        } else {
            System.out.println("Lisp was not replaced");
        }
        for (String key : languages.keySet()) {
            System.out.println(key + " : " + languages.get(key));
        }
    }
}

輸出結果：

Lisp was not replaced

Java : a compiled high level, object-oriented, platform independent language.

BASIC : Beginners All Purposes Symbolic Instruction Code.

Lisp : Therein lies madness

Python : an interpreted, object-oriented, high-level programming language.

Algol : an algorithmic language

import java.util.HashMap;
import java.util.Map;
public class MapProgram {
    public static void main(String[] args) {
        Map<String, String> languages = new HashMap<>();
        languages.put("Java", " a compiled high level, object-oriented, platform independent language.");
        languages.put("Python", " an interpreted, object-oriented, high-level programming language.");
        languages.put("Algol", " an algorithmic language");
        languages.put("BASIC", " Beginners All Purposes Symbolic Instruction Code.");
        languages.put("Lisp", " Therein lies madness");
        if (languages.replace("Lisp", " Therein lies madness", "a functional programming language with imperative features.")) {
            System.out.println("Lisp replace");
        } else {
            System.out.println("Lisp was not replaced");
        }
        for (String key : languages.keySet()) {
            System.out.println(key + " : " + languages.get(key));
        }
    }
}

輸出結果：

Lisp replace

Java : a compiled high level, object-oriented, platform independent language.

BASIC : Beginners All Purposes Symbolic Instruction Code.

Lisp : a functional programming language with imperative features.

Python : an interpreted, object-oriented, high-level programming language.

Algol : an algorithmic language

這方法就能確定符合目標的值，才能被更新

避免了更新到非目標值的機會

Tim 舉例說：像是更新結婚後的姓氏，先確保原姓氏符合，符合了再更新，這樣就避免出現意外覆蓋的情況

本篇使用代碼