构造函数和数组列表 [重复]
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【中文标题】构造函数和数组列表 [重复]【英文标题】：Constructor and Array Lists [duplicate] 【发布时间】：2019-05-03 04:46:25 【问题描述】：我正在尝试创建一个（美国总统的）测验程序。我想创建一个数组列表（总统编号、总统姓名、起始年份、去年）。
由于某种原因，我的构造函数不起作用，我不得不构建另一个方法，但第二种方法导致 ***Error （这是我第一次使用这个网站来解决问题，我想我来到了给定站点名称的正确位置）
这是我的代码：
public class Presidents
public int presidentNumber;
public String presidentName;
public int startingYear;
public int endingYear;
public Presidents(int PresidentNumber, String NameOfPresident, int StartingYear, int EndingYear)
this.presidentNumber = PresidentNumber;
this.presidentName = NameOfPresident;
this.startingYear = StartingYear;
this.endingYear = EndingYear;
public Presidents()
public Presidents [] PresidentList ()
Presidents [] president = new Presidents [45];
president [0] = Presidents(1, "George Washington", 1789, 1797);
president [1] = Presidents(2, "John Adams", 1797, 1801);
president [2] = Presidents(3, "Thomas Jefferson", 1801, 1809);
president [3] = Presidents(4, "James Madison", 1809, 1817);
president [4] = Presidents(5, "James Monroe", 1817, 1825);
president [5] = Presidents(6, "John Quincy Adams", 1825, 1829);
president [6] = Presidents(7, "Andrew Jackson", 1829, 1837);
president [7] = Presidents(8, "Martin Van Buren", 1837, 1841);
president [8] = Presidents(9, "William Henry Harrison", 1841, 1841);
president [9] = Presidents(10, "John Tyler", 1841, 1845);
president [10] = Presidents(11, "James K. Polk", 1845, 1849);
president [11] = Presidents(12, "Zachary Taylor", 1849, 1850);
president [12] = Presidents(13, "Millard Fillmore", 1850, 1853);
president [13] = Presidents(14, "Franklin Pierce", 1853, 1857);
president [14] = Presidents(15, "James Buchanan", 1857, 1861);
president [15] = Presidents(16, "Abraham Lincoln", 1861, 1865);
president [16] = Presidents(17, "Andrew Johnson", 1865, 1869);
president [17] = Presidents(18, "Ulysses S. Grant", 1869, 1877);
president [18] = Presidents(19, "Rutherford B. Hayes", 1877, 1881);
president [19] = Presidents(20, "James A. Garfield", 1881, 1881);
president [20] = Presidents(21, "Chester A. Arthur", 1881, 1885);
president [21] = Presidents(22, "Grover Cleveland", 1885, 1889);
president [22] = Presidents(23, "Benjamin Harrison", 1889, 1893);
president [23] = Presidents(24, "Grover Cleveland", 1893, 1897);
president [24] = Presidents(25, "William McKinley", 1897, 1901);
president [25] = Presidents(26, "Theodore Roosevelt", 1901, 1909);
president [26] = Presidents(27, "William Howard Taft", 1909, 1913);
president [27] = Presidents(28, "Woodrow Wilson", 1913, 1921);
president [28] = Presidents(29, "Warren G. Harding", 1921, 1923);
president [29] = Presidents(30, "Calvin Coolidge", 1923, 1929);
president [30] = Presidents(31, "Herbert Hoover", 1929, 1933);
president [31] = Presidents(32, "Franklin D. Roosevelt", 1933, 1945);
president [32] = Presidents(33, "Harry S. Truman", 1945, 1953);
president [33] = Presidents(34, "Dwight D. Eisenhower", 1953, 1961);
president [34] = Presidents(35, "John F. Kennedy", 1961, 1963);
president [35] = Presidents(36, "Lyndon B. Johnson", 1963, 1969);
president [36] = Presidents(37, "Richard Nixon", 1969, 1974);
president [37] = Presidents(38, "Gerald Ford", 1974, 1977);
president [38] = Presidents(39, "Jimmy Carter", 1977, 1981);
president [39] = Presidents(40, "Ronald Reagan", 1981, 1989);
president [40] = Presidents(41, "George H. W. Bush", 1989, 1993);
president [41] = Presidents(42, "Bill Clinton", 1993, 2001);
president [42] = Presidents(43, "George W. Bush", 2001, 2009);
president [43] = Presidents(44, "Barack Obama", 2009, 2017);
president [44] = Presidents(45, "Donald Trump", 2017, 2018);
return president;
private Presidents Presidents(int i, String string, int j, int k)
return Presidents (i, string, j, k);
import java.util.Scanner;
import java.util.Random;
public class PresidentsQuiz
public static void main (String [] args)
System.out.println("Do you know the 45 presidents of the United States? Enter a number between 1 and 45");
Scanner kb = new Scanner (System.in);
int input = kb.nextInt();
while (input < 1 || input > 45)
System.out.println("Enter a number between 1 and 45");
input = kb.nextInt();
Presidents president = new Presidents ();
Presidents [] presidentList = president.PresidentList();
System.out.println(presidentList [0]);
kb.close();
【问题讨论】：
作为旁注，我建议查看 Java 命名约定 here。遵循这些约定将有助于使您的代码更具可读性。 【参考方案1】：你这里有一个方法：
public Presidents [] PresidentList ()
Presidents [] president = new Presidents [45];
president [0] = Presidents(1, "George Washington", 1789, 1797);
president [1] = Presidents(2, "John Adams", 1797, 1801);
president [2] = Presidents(3, "Thomas Jefferson", 1801, 1809);
president [3] = Presidents(4, "James Madison", 1809, 1817);
president [4] = Presidents(5, "James Monroe", 1817, 1825);
//...
return president;
但是每次调用：
president [...] = Presidents(...);
将调用：
private Presidents Presidents(int i, String string, int j, int k)
return Presidents (i, string, j, k);
调用相同参数的相同方法，将永远递归，导致堆栈溢出错误。
没有一个方法与类调用相同的东西，并返回一些东西。这是为构造函数保留的。你已经有一个构造函数接受正确的参数，所以只需删除该方法：
private Presidents Presidents(int i, String string, int j, int k)
return Presidents (i, string, j, k);
然后在你的方法中你需要添加new关键字：
president [0] = new Presidents(1, "George Washington", 1789, 1797);
^--- Here
【讨论】：
当我尝试使用构造函数时出现错误。它说“Presidents(int, String, int, int) 方法未定义总统类型”，它迫使我创建那个方法（你告诉我删除的那个）。【参考方案2】：您应该在初始化数组时创建 Presidents 类的新对象，因为数组应该存储 Presidents 类型的对象。相反，您只调用了类的构造函数。而不是这样做：
president [0] = Presidents(1, "George Washington", 1789, 1797);
你应该这样做：
president [0] = new Presidents(1, "George Washington", 1789, 1797);
对其余的数组索引也这样做。而这个方法：
private Presidents Presidents(int i, String string, int j, int k)
return Presidents (i, string, j, k);
这是定义方法的错误方式。该方法不应与类同名。只有构造函数应该拥有它。此外，如果您希望此方法返回 Presidents 类的对象，则返回语句应如下所示：
return Presidents (i, string, j, k);
但是这个方法是没有意义的，因为你已经有 PresidentList() 方法返回数组。话虽如此，最好将类中的变量设为私有并定义 getter 方法来访问它们。无论如何，我已经对您的代码进行了一些修改，这应该可以工作：
public class Presidents
public int presidentNumber;
public String presidentName;
public int startingYear;
public int endingYear;
Presidents [] president;
public Presidents(int PresidentNumber, String NameOfPresident, int StartingYear, int EndingYear)
this.presidentNumber = PresidentNumber;
this.presidentName = NameOfPresident;
this.startingYear = StartingYear;
this.endingYear = EndingYear;
public Presidents()
public Presidents [] PresidentList ()
president= new Presidents [45];
president [0] = new Presidents(1, "George Washington", 1789, 1797);
president [1] = new Presidents(2, "John Adams", 1797, 1801);
president [2] = new Presidents(3, "Thomas Jefferson", 1801, 1809);
president [3] = new Presidents(4, "James Madison", 1809, 1817);
president [4] = new Presidents(5, "James Monroe", 1817, 1825);
president [5] = new Presidents(6, "John Quincy Adams", 1825, 1829);
president [6] = new Presidents(7, "Andrew Jackson", 1829, 1837);
president [7] = new Presidents(8, "Martin Van Buren", 1837, 1841);
president [8] = new Presidents(9, "William Henry Harrison", 1841, 1841);
president [9] = new Presidents(10, "John Tyler", 1841, 1845);
president [10] = new Presidents(11, "James K. Polk", 1845, 1849);
president [11] = new Presidents(12, "Zachary Taylor", 1849, 1850);
president [12] = new Presidents(13, "Millard Fillmore", 1850, 1853);
president [13] = new Presidents(14, "Franklin Pierce", 1853, 1857);
president [14] = new Presidents(15, "James Buchanan", 1857, 1861);
president [15] = new Presidents(16, "Abraham Lincoln", 1861, 1865);
president [16] = new Presidents(17, "Andrew Johnson", 1865, 1869);
president [17] = new Presidents(18, "Ulysses S. Grant", 1869, 1877);
president [18] =new Presidents(19, "Rutherford B. Hayes", 1877, 1881);
president [19] =new Presidents(20, "James A. Garfield", 1881, 1881);
president [20] =new Presidents(21, "Chester A. Arthur", 1881, 1885);
president [21] =new Presidents(22, "Grover Cleveland", 1885, 1889);
president [22] =new Presidents(23, "Benjamin Harrison", 1889, 1893);
president [23] =new Presidents(24, "Grover Cleveland", 1893, 1897);
president [24] =new Presidents(25, "William McKinley", 1897, 1901);
president [25] =new Presidents(26, "Theodore Roosevelt", 1901, 1909);
president [26] =new Presidents(27, "William Howard Taft", 1909, 1913);
president [27] =new Presidents(28, "Woodrow Wilson", 1913, 1921);
president [28] =new Presidents(29, "Warren G. Harding", 1921, 1923);
president [29] =new Presidents(30, "Calvin Coolidge", 1923, 1929);
president [30] =new Presidents(31, "Herbert Hoover", 1929, 1933);
president [31] =new Presidents(32, "Franklin D. Roosevelt", 1933, 1945);
president [32] =new Presidents(33, "Harry S. Truman", 1945, 1953);
president [33] =new Presidents(34, "Dwight D. Eisenhower", 1953, 1961);
president [34] =new Presidents(35, "John F. Kennedy", 1961, 1963);
president [35] =new Presidents(36, "Lyndon B. Johnson", 1963, 1969);
president [36] =new Presidents(37, "Richard Nixon", 1969, 1974);
president [37] =new Presidents(38, "Gerald Ford", 1974, 1977);
president [38] =new Presidents(39, "Jimmy Carter", 1977, 1981);
president [39] =new Presidents(40, "Ronald Reagan", 1981, 1989);
president [40] =new Presidents(41, "George H. W. Bush", 1989, 1993);
president [41] =new Presidents(42, "Bill Clinton", 1993, 2001);
president [42] =new Presidents(43, "George W. Bush", 2001, 2009);
president [43] =new Presidents(44, "Barack Obama", 2009, 2017);
president [44] =new Presidents(45, "Donald Trump", 2017, 2018);
return president;
还有主课：
import java.util.Scanner;
import java.util.Random;
public class PresidentsQuiz
public static void main (String [] args)
System.out.println("Do you know the 45 presidents of the United States? Enter a number between 1 and 45");
Scanner kb = new Scanner (System.in);
int input = kb.nextInt();
while (input < 1 || input > 45)
System.out.println("Enter a number between 1 and 45");
input = kb.nextInt();
Presidents president = new Presidents ();
Presidents [] presidentList = president.PresidentList();
//System.out.println(presidentList [0].presidentName);
for(int i=0;i<input;i++)
Presidents p = presidentList[i];
System.out.println(p.presidentNumber+", "+p.presidentName+", "+p.startingYear+", "+p.endingYear);
kb.close();
我认为您试图打印所有总统，直到您输入的数字。如果没有，您可以删除 main 方法中的 for 循环。
【讨论】：
【参考方案3】：解决方案
必须将创建总统列表的逻辑移至主类原因
逻辑是程序的职责 为各个类分配职责至关重要 一个对象可以负责使用构造函数创建它自己的单个实例。但不负责使用其他实例方法创建自己的实例列表。 从您的代码中，您将拥有一个字段中没有任何值的总裁对象。该虚拟实例负责创建真实的总统列表。代码
public class Presidents
public int presidentNumber;
public String presidentName;
public int startingYear;
public int endingYear;
public Presidents(int PresidentNumber, String NameOfPresident, int StartingYear, int EndingYear)
this.presidentNumber = PresidentNumber;
this.presidentName = NameOfPresident;
this.startingYear = StartingYear;
this.endingYear = EndingYear;
public Presidents()
import java.util.Scanner;
import java.util.Random;
public class PresidentsQuiz
public static void main (String [] args)
System.out.println("Do you know the 45 presidents of the United States? Enter a number between 1 and 45");
Scanner kb = new Scanner (System.in);
int input = kb.nextInt();
while (input < 1 || input > 45)
System.out.println("Enter a number between 1 and 45");
input = kb.nextInt();
Presidents [] president = new Presidents [45];
president [0] = new Presidents(1, "George Washington", 1789, 1797);
president [1] = new Presidents(2, "John Adams", 1797, 1801);
president [2] = new Presidents(3, "Thomas Jefferson", 1801, 1809);
president [3] = new Presidents(4, "James Madison", 1809, 1817);
president [4] = new Presidents(5, "James Monroe", 1817, 1825);
president [5] = new Presidents(6, "John Quincy Adams", 1825, 1829);
president [6] = new Presidents(7, "Andrew Jackson", 1829, 1837);
president [7] = new Presidents(8, "Martin Van Buren", 1837, 1841);
president [8] = new Presidents(9, "William Henry Harrison", 1841, 1841);
president [9] = new Presidents(10, "John Tyler", 1841, 1845);
president [10] = new Presidents(11, "James K. Polk", 1845, 1849);
president [11] = new Presidents(12, "Zachary Taylor", 1849, 1850);
president [12] = new Presidents(13, "Millard Fillmore", 1850, 1853);
president [13] = new Presidents(14, "Franklin Pierce", 1853, 1857);
president [14] = new Presidents(15, "James Buchanan", 1857, 1861);
president [15] = new Presidents(16, "Abraham Lincoln", 1861, 1865);
president [16] = new Presidents(17, "Andrew Johnson", 1865, 1869);
president [17] = new Presidents(18, "Ulysses S. Grant", 1869, 1877);
president [18] = new Presidents(19, "Rutherford B. Hayes", 1877, 1881);
president [19] = new Presidents(20, "James A. Garfield", 1881, 1881);
president [20] = new Presidents(21, "Chester A. Arthur", 1881, 1885);
president [21] = new Presidents(22, "Grover Cleveland", 1885, 1889);
president [22] = new Presidents(23, "Benjamin Harrison", 1889, 1893);
president [23] = new Presidents(24, "Grover Cleveland", 1893, 1897);
president [24] = new Presidents(25, "William McKinley", 1897, 1901);
president [25] = new Presidents(26, "Theodore Roosevelt", 1901, 1909);
president [26] = new Presidents(27, "William Howard Taft", 1909, 1913);
president [27] = new Presidents(28, "Woodrow Wilson", 1913, 1921);
president [28] = new Presidents(29, "Warren G. Harding", 1921, 1923);
president [29] = new Presidents(30, "Calvin Coolidge", 1923, 1929);
president [30] = new Presidents(31, "Herbert Hoover", 1929, 1933);
president [31] = new Presidents(32, "Franklin D. Roosevelt", 1933, 1945);
president [32] = new Presidents(33, "Harry S. Truman", 1945, 1953);
president [33] = new Presidents(34, "Dwight D. Eisenhower", 1953, 1961);
president [34] = new Presidents(35, "John F. Kennedy", 1961, 1963);
president [35] = new Presidents(36, "Lyndon B. Johnson", 1963, 1969);
president [36] = new Presidents(37, "Richard Nixon", 1969, 1974);
president [37] = new Presidents(38, "Gerald Ford", 1974, 1977);
president [38] = new Presidents(39, "Jimmy Carter", 1977, 1981);
president [39] = new Presidents(40, "Ronald Reagan", 1981, 1989);
president [40] = new Presidents(41, "George H. W. Bush", 1989, 1993);
president [41] = new Presidents(42, "Bill Clinton", 1993, 2001);
president [42] = new Presidents(43, "George W. Bush", 2001, 2009);
president [43] = new Presidents(44, "Barack Obama", 2009, 2017);
president [44] = new Presidents(45, "Donald Trump", 2017, 2018);
System.out.println(presidentList [0]);
kb.close();
【讨论】：
【参考方案4】：当您在列表中的特定位置进行分配时，您需要实例化一个新的总裁对象。
这里你的方法：-
private Presidents Presidents(int i, String string, int j, int k)
//does not Instantiate a new object
return Presidents (i, string, j, k);
-这里它不实例化Presidents 类的新对象。它调用相同的总统方法并发生递归调用。
所以这里你需要用new 关键字来实例化你的方法。
return new Presidents (i, string, j, k);
它指向类，构造函数将被调用。
【讨论】：
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