Java重要知识点（继承、多态、接口，异常，工具，测试）

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

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 them
• DbUnit 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