Java重要知识点（继承、多态、接口，异常，工具，测试）

2019-11-11 06:50:29

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重载、多态、接口

多态调用的顺序

当父类，和子类有Static时，先初始化Static，再初始化子类的Static，再初始化父类的其他成员变量－>父类构造方法－>子类其他成员变量－>子类的构造方法。

重写 hashcode equals compareto

Employee.java

import java.util.Date;import java.util.GregorianCalendar;import java.util.Objects;public class Employee implements Comparable<Employee> { PRivate String name; private double salary; private Date hireDay; public Employee(String n, double s, int year, int month, int day) { name = n; salary = s; GregorianCalendar calendar = new GregorianCalendar(year, month - 1, day); hireDay = calendar.getTime(); } public String getName() { return name; } public double getSalary() { return salary; } public Date getHireDate() { return hireDay; } public boolean equals(Object Otherobject) { if (this == Otherobject) return true; if (Otherobject == null) return false; // if(! Otherobject instanceof Employee) return false; if (getClass() != Otherobject.getClass()) return false; Employee other = (Employee) Otherobject; return Objects.equals(name, other.name) && salary == other.salary && Objects.equals(hireDay, other.hireDay); } public int hashCode() { return Objects.hash(name, salary); } public String toString() { return getClass().getName() + "name=" + name + ",salary=" + salary + ",hireDay=" + hireDay; } @Override public int compareTo(Employee other) { // if(getClass() != other.getClass()) throw new ClassCastException(); return Double.compare(salary, other.salary); }}

Manager.java

public class Manager extends Employee { private double bonus; public Manager(String n, double s, int year, int month, int day) { super(n, s, year, month, day); bonus = 0; } public void setBonus(double b) { bonus = b; } @Override public double getSalary() { double baseSalary = super.getSalary(); return bonus + baseSalary; } @Override public boolean equals(Object Otherobject) { if (!super.equals(Otherobject)) return false; Manager other = (Manager) Otherobject; return bonus == other.bonus; } @Override public int hashCode() { return super.hashCode() + 17 * new Double(bonus).hashCode(); } @Override public String toString() { return super.toString() + ",bonus=" + bonus; }}

异常

涉及return finally 以及异常抛出的问题

这里写图片描述

各个异常出现的情况

package exp;import java.awt.Font;import java.awt.FontFormatException;import java.awt.Graphics;import java.awt.Image;import java.awt.Toolkit;import java.beans.Statement;import java.io.EOFException;import java.io.File;import java.io.FileInputStream;import java.io.FileNotFoundException;import java.io.IOException;import java.nio.ByteBuffer;import java.sql.Connection;import java.sql.DriverManager;import java.util.Stack;public class AllExcept { public static void main(String[] args) throws Exception { // 01 空指针异常 java.lang.NullPointerException try { int[] s = null; int i = s.length; } catch (Exception e) { System.out.println("1、" + e.getClass().getName() + "异常"); } // 02数组越界 java.lang.ArrayIndexOutOfBoundsException异常 try { int[] s1 = new int[2]; s1[2] = 5; } catch (Exception e) { System.out.println("2、" + e.getClass().getName() + "异常"); } // 03 错误类型的对象存储到一个对象数组时抛出的异常 java.lang.ArrayStoreException异常 try { Object x[] = new String[3]; x[0] = new Integer(0); } catch (Exception e) { System.out.println("3、" + e.getClass().getName() + "异常"); } // 04 除数为0异常 java.lang.ArithmeticException异常 try { int a = 0; int b = 10; b /= a; } catch (Exception e) { System.out.println("4、" + e.getClass().getName() + "异常"); } // 05 数据类型转换异常 java.lang.NumberFormatException异常 try { String s2 = "19k"; int n = Integer.parseInt(s2); } catch (Exception e) { System.out.println("5、" + e.getClass().getName() + "异常"); } // 06 超过字符串添加长度异常 java.lang.StringIndexOutOfBoundsException异常 try { StringBuffer buffer = new StringBuffer(); buffer.charAt(5); } catch (Exception e) { System.out.println("6、" + e.getClass().getName() + "异常"); } // 07 数组大小为负值异常。 java.lang.NegativeArraySizeException异常 try { int i = -1; int[] s = new int[i]; } catch (Exception e) { System.out.println("7、" + e.getClass().getName() + "异常"); } // 08 正则表达式异常 java.util.regex.PatternSyntaxException异常 try { String a = "//";// 错误的正则表达式 String a1 = "//.";// 正确的正则表达式 String[] x5 = "abc.cde".split(a1); x5 = "abc.cde".split(a); } catch (Exception e) { System.out.println("8、" + e.getClass().getName() + "异常"); } // 09 空栈异常 java.util.EmptyStackException异常 try { Stack s = new Stack(); s.pop(); } catch (Exception e) { System.out.println("9、" + e.getClass().getName() + "异常"); } // 10 找不到类异常 java.lang.ClassNotFoundException异常 try { Object s = Class.forName("ClassName"); } catch (Exception e) { System.out.println("10、" + e.getClass().getName() + "异常"); } // 11非法参数异常 // 12 字体格式错误异常 try { Font f = new Font("楷体", Font.BOLD, 7); f.createFont(2, new File("1.txt")); } catch (FontFormatException e) { System.out.println("11、" + e.getClass().getName()); } catch (IllegalArgumentException e) { System.out.println("12、" + e.getClass().getName() + "异常"); } // 13不支持画图异常 try { Image img = Toolkit.getDefaultToolkit().getImage("Images/1.jpg"); Graphics g = img.getGraphics(); g.drawRect(-1, -1, -1, -1); } catch (Exception e) { System.out.println("13、" + e.getClass().getName() + "异常"); } // 14 BufferOverflowException try { int cap = 2; ByteBuffer bf = ByteBuffer.allocate(cap); for (int i = 0; i < cap; i++) { bf.put((byte) i); } bf.put((byte) 10); bf.getInt(9); } catch (Exception e) { System.out.println("14、" + e.getClass().getName() + "异常"); } // 15文件找不到异常 // 16.当发生某种 I/O 异常时，抛出IOException // 17.文件已结束异常EOFException try { File f = new File("E://1.txt"); byte[] buff = new byte[2]; FileInputStream file = new FileInputStream(f); int bytes = file.read(buff, 0, 2); System.out.println(new String(buff, 0, 0, bytes)); file.close(); } catch (FileNotFoundException e) { System.out.println("15、" + e.getClass().getName() + "异常"); } catch (EOFException e) { System.out.println("16、" + e.getClass().getName() + "异常"); } catch (IOException e) { System.out.println("17、" + e.getClass().getName() + "异常"); } // 18 ClassCastException异常 try { Object s = new Integer(0); System.out.println((String) s); } catch (Exception e) { System.out.println("18、" + e.getClass().getName() + "异常"); } // 19 BufferUnderflowException异常 try { ByteBuffer buf = ByteBuffer.allocate(50); buf.flip(); byte[] byt = new byte[100]; buf.get(byt); new RuntimeException(); } catch (Exception e) { System.out.println("19、" + e.getClass().getName() + "异常"); } // 20 java.sql.SQLException异常 try { String sql = "select * from Table";// sql语句错误 Connection conn = DriverManager.getConnection("com.MySQL.jdbc.Driver"); Statement smt = (Statement) conn.createStatement(); smt.execute(); } catch (Exception e) { System.out.println("20、" + e.getClass().getName() + "异常"); } }}

工具

git svn 的区别和常用命令

Maven依赖中的scope

测试

JUnit

Die.java

public class Die{ private int faceValue; private int numofsides; public Die(){ faceValue = 0; numofsides = 6; } public Die(int v){ numofsides = v; } public int numOfSides(){ return numofsides; } public int topFace(){ return faceValue; } public void roll() { faceValue = (int) (1 + Math.random() * numofsides ); }}

TestDie.java

import org.junit.After;import org.junit.Assert;import org.junit.Test;import org.junit.Before;/** * Unit test for simple App. */public class TestDie { private Die die1,die2; @Before public void setUp() { die1 = new Die(); die2 = new Die(10); die1.roll(); die2.roll(); } @Test public void testdie1numofsides() { Assert.assertEquals(6, die1.numOfSides()); } @Test public void testdie1Face() { Assert.assertTrue(die1.topFace() >= 1 && die1.topFace() <= 6); } @Test public void testdie2numofsides() { Assert.assertEquals(10, die2.numOfSides()); } @Test public void testdie2Face() { Assert.assertTrue(die2.topFace() >= 1 && die2.topFace() <= 10); } @After public void tearDown() { }}

实现 assertEquals aseertSame

assertEquals.java

当expected和actual都是String类型时，ComparisonFailure还会找出是前后相同的串，并用[Different String]标明那些不相同的字符串，也就是expectedToString和actualToString的格式将会变成：…${sameString}[${differentString}]${sameString}…。其中“…”只会在相同的字符串太长的情况下才会出现，这个长度标准目前（JUnit4.10）是20个字符。具体实现参考ComparisonFailure类，它继承自AssertionError，这里不再展开。

/** * Asserts that two objects are equal. If they are not, an * {@link AssertionError} is thrown with the given message. If * <code>expected</code> and <code>actual</code> are <code>null</code>, * they are considered equal. * * @param message * the identifying message for the {@link AssertionError} (<code>null</code> * okay) * @param expected * expected value * @param actual * actual value */ static public void assertEquals(String message, Object expected, Object actual) { if (expected == null && actual == null) return; if (expected != null && isEquals(expected, actual)) return; else if (expected instanceof String && actual instanceof String) { String cleanMessage= message == null ? "" : message; throw new ComparisonFailure(cleanMessage, (String) expected, (String) actual); } else failNotEquals(message, expected, actual); } private static boolean isEquals(Object expected, Object actual) { return expected.equals(actual); } /** * Asserts that two objects are equal. If they are not, an * {@link AssertionError} without a message is thrown. If * <code>expected</code> and <code>actual</code> are <code>null</code>, * they are considered equal. * * @param expected * expected value * @param actual * the value to check against <code>expected</code> */ static public void assertEquals(Object expected, Object actual) { assertEquals(null, expected, actual); }

aseertSame.java

/** * Asserts that two objects refer to the same object. If they are not, an * {@link AssertionError} is thrown with the given message. * * @param message * the identifying message for the {@link AssertionError} (<code>null</code> * okay) * @param expected * the expected object * @param actual * the object to compare to <code>expected</code> */ static public void assertSame(String message, Object expected, Object actual) { if (expected == actual) return; failNotSame(message, expected, actual); } /** * Asserts that two objects refer to the same object. If they are not the * same, an {@link AssertionError} without a message is thrown. * * @param expected * the expected object * @param actual * the object to compare to <code>expected</code> */ static public void assertSame(Object expected, Object actual) { assertSame(null, expected, actual); }

其他

hamcrest 可以用来增强 JUnit 中的 assert 功能Cobertura is a free Java tool that calculates the percentage of code accessed by tests.stub & mock. EasyMock has been the first dynamic Mock Object generator, relieving users of hand-writing Mock Objects, or generating code for themDbUnit is a JUnit extension (also usable with Ant) targeted at database-driven projects that,among other things, puts your database into a known state between test runs

本文转自：http://blog.csdn.net/u011239443/article/details/53150159

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