今天要學的是 Packages
如果我們沒有指定 Package
只打了這樣
Node node = null;
系統就會報錯
Cannot resolve symbol ‘Node’
因為同名的太多 系
import org.w3c.dom.Node;
import sun.tools.tree.Node;public class Main {
public static void main(String[] args) {
sun.tools.tree.Node node = null;
org.w3c.dom.Node node321 = null; }
}
系統不知道你要用那個
我們就要在上方 import
import jdk.nashorn.internal.ir.Node;
也可以在前面指定
org.w3c.dom.Node node = null;
這樣不用 import 系統也能接受
但是不能 import 兩個同名
import org.w3c.dom.Node;
import sun.tools.tree.Node;public class Main {
public static void main(String[] args) {
sun.tools.tree.Node node = null;
org.w3c.dom.Node node321 = null; }
}
系統就會在 line2 報錯
‘org.w3c.dom.Node’ is already defined in a single-type import
public class Main {
public static void main(String[] args) { sun.tools.tree.Node node = null;
org.w3c.dom.Node node321 = null; }
}
雖然移除掉 import intellij 沒有提示報錯
但一執行就錯誤
java: package sun.tools.tree does not exist
導出寫好的 package
點 project structure（左上 file 選單跟右上角圖片都有） → 左側點選第五個的 Arifacts → 點選旁邊上方 + 號 →
點選 jar 第二個 from modules with dependencies → 確認是否為要導出的 package ，若有指定第二行 Main Class 還能選輸出位置也可不指定，確認好點 ok →
之後點上方 Build → Artifacts → build 就完成導出啦
沒指定位置的話 就是在 out 裡面
如：java-the-complete-java-developer-course/test/out/artifacts/test_jar
如果要用命令行執行 可以到那個位置後
java -jar 檔案名
就可以在視窗內如 Terminal 執行程序了
導入 package
一樣打開 project structure 點選第三個的 Libraries 按旁邊 + 導入即可
小挑戰：
Creamain() method of your new project.te a suitably named package containing a class called Series
with the following static methods:
nSum(int n) returns the sum of all numbers from 0 to n. The first 10 numbers are:
0, 1, 3, 6, 10, 15, 21, 28, 36, 45, 55.
factorial(int n) returns the product of all numbers from 1 to n
i.e. 1 * 2 * 3 * 4 … * (n — 1) * n.
The first 10 factorials are:
0, 1, 2, 6, 24, 120, 720, 5040, 40320, 362880, 3628800.
fibonacci(n) returns the nth Fibonacci number. These are defined as:
f(0) = 0
f(1) = 1
f(n) = f(n-1) + f(n-2)
(so f(2) is also 1. The first 10 fibonacci numbers are:
0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55.
When you have tested your functions, delete the Main class and
produce a jar file.
Create a new project and add your Series library, then test the
three methods in the
解法參考：
Series.java
package com.mylibrary;
public class Series {
public static int nSum(int n) {
// int num = 0;
// for (int i = 1; i <= n; i++) {
// num += i - 1;
// }
// return num;
if (n == 0) {
return 0;
} else
return n + nSum(n - 1); } public static int factorial(int n) {
// int num = 1;
// for (int i = 1; i <= n; i++) {
// num *= i;
// }
// return num;
if (n == 0) {
return 0;
} else if (n == 1) {
return 1;
} else
return n * factorial(n - 1);
}
public static int fibonacci(int n) {
if (n == 0) {
return 0;
} else if (n == 1) {
return 1;
} else {
return fibonacci(n - 1) + fibonacci(n - 2);
} }}
MainSeries.java
import com.mylibrary.Series;public class MainSeries {
public static void main(String[] args) {
for (int i = 0; i <= 10; i++) {
System.out.println(Series.nSum(i));
}
System.out.println("===============");
for (int i = 0; i <= 10; i++) {
System.out.println(Series.factorial(i));
}
System.out.println("===============");
for (int i = 0; i <= 10; i++) {
System.out.println(Series.fibonacci(i));
}
}
}
Tim Version:
package com.timbuchalka.mylibrary;/**
* Created by dev on 5/11/2015.
*/
public class Series { public static long nSum(int n) {
return (n * (n + 1)) /2;
} public static long factorial(int n) {
if(n == 0) {
return 0;
}
long fact = 1;
for (int i=1; i <= n; i++) {
fact *= i;
} return fact;
} public static long fibonacci(int n) {
if(n == 0) {
return 0;
} else if(n == 1) {
return 1;
}
long nMinus1 = 1;
long nMinus2 = 0;
long fib = 0;
for(int i= 1; i<n; i++) {
fib = (nMinus2 + nMinus1);
nMinus2 = nMinus1;
nMinus1 = fib;
}
return fib;
}
}