JAVA 工具類整理

Guava Optional類：Optional用於包含非空對象的不可變對象。 Optional對象，用於不存在值表示null。這個類有各種實用的方法，以方便代碼來處理爲可用或不可用，而不是檢查null值。

查看代碼

import com.google.common.base.Optional;

public class GuavaTester {
   public static void main(String args[]){
      GuavaTester guavaTester = new GuavaTester();

      Integer value1 =  null;
      Integer value2 =  new Integer(10);
      //Optional.fromNullable - allows passed parameter to be null.
      Optional<Integer> a = Optional.fromNullable(value1);
      //Optional.of - throws NullPointerException if passed parameter is null
      Optional<Integer> b = Optional.of(value2);		

      System.out.println(guavaTester.sum(a,b));
   }

   public Integer sum(Optional<Integer> a, Optional<Integer> b){
      //Optional.isPresent - checks the value is present or not
      System.out.println("First parameter is present: " + a.isPresent());

      System.out.println("Second parameter is present: " + b.isPresent());

      //Optional.or - returns the value if present otherwise returns
      //the default value passed.
      Integer value1 = a.or(new Integer(0));	

      //Optional.get - gets the value, value should be present
      Integer value2 = b.get();

      return value1 + value2;
   }	
}
//更多請閱讀：https://www.yiibai.com/guava/guava_optional_class.html

Guava Preconditions類：Preconditions提供靜態方法來檢查方法或構造函數，被調用是否給定適當的參數。它檢查的先決條件。其方法失敗拋出IllegalArgumentException。

查看代碼

import com.google.common.base.Preconditions;

public class GuavaTester {

   public static void main(String args[]){
      GuavaTester guavaTester = new GuavaTester();
      try {
         System.out.println(guavaTester.sqrt(-3.0));
      }catch(IllegalArgumentException e){
         System.out.println(e.getMessage());
      }
      try {
         System.out.println(guavaTester.sum(null,3));
      }catch(NullPointerException e){
         System.out.println(e.getMessage());
      }
      try {
         System.out.println(guavaTester.getValue(6));
      }catch(IndexOutOfBoundsException e){
         System.out.println(e.getMessage());
      }
   }

   public double sqrt(double input) throws IllegalArgumentException {
      Preconditions.checkArgument(input > 0.0,
         "Illegal Argument passed: Negative value %s.", input);
      return Math.sqrt(input);
   }	

   public int sum(Integer a, Integer b){
      a = Preconditions.checkNotNull(a,
         "Illegal Argument passed: First parameter is Null.");
      b = Preconditions.checkNotNull(b,
         "Illegal Argument passed: Second parameter is Null.");
      return a+b;
   }

   public int getValue(int input){
      int[] data = {1,2,3,4,5};
      Preconditions.checkElementIndex(input,data.length,
         "Illegal Argument passed: Invalid index.");
      return 0;
   }
}

Guava Ordering類：Ordering(排序)可以被看作是一個豐富的比較具有增強功能的鏈接，多個實用方法，多類型排序功能等。

查看代碼

package org.example;

import com.google.common.collect.Ordering;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class GuavaTester {
    public static void main(String args[]){
        List<Integer> numbers = new ArrayList<Integer>();
        numbers.add(5);
        numbers.add(2);
        numbers.add(15);
        numbers.add(51);
        numbers.add(53);
        numbers.add(35);
        numbers.add(45);
        numbers.add(32);
        numbers.add(43);
        numbers.add(16);

        Ordering ordering = Ordering.natural();
        System.out.println("Input List: "+numbers);

        Collections.sort(numbers,ordering );
        System.out.println("SortedList: "+numbers);

        System.out.println("======================");
        System.out.println("List is sorted: " + ordering.isOrdered(numbers));
        System.out.println("Minimum: " + ordering.min(numbers));
        System.out.println("Maximum: " + ordering.max(numbers));

        Collections.sort(numbers,ordering.reverse());
        System.out.println("Reverse: " + numbers);

        numbers.add(null);
        System.out.println("Null added to Sorted List: "+numbers);

        Collections.sort(numbers,ordering.nullsFirst());
        System.out.println("Null first Sorted List:    "+numbers);
        Collections.sort(numbers,ordering.nullsLast());
        System.out.println("Null last Sorted List:     "+numbers);
        System.out.println("======================");

        List<String> names = new ArrayList<String>();
        names.add("Ram");
        names.add("Shyam");
        names.add("Mohan");
        names.add("Sohan");
        names.add("Ramesh");
        names.add("Suresh");
        names.add("Naresh");
        names.add("Mahesh");
        names.add(null);
        names.add("Vikas");
        names.add("Deepak");

        System.out.println("Another List: "+names);

        Collections.sort(names,ordering.nullsFirst().reverse());
        System.out.println("Null first then reverse sorted list: "+names);
    }
}

Guava Objects類：Objects類提供適用於所有對象，如equals, hashCode等輔助函數

package org.example;

import com.google.common.base.MoreObjects;
import com.google.common.base.Objects;

public class GuavaTester {
    public static void main(String args[]){
        Student s1 = new Student("Mahesh", "Parashar", 1, "VI");
        Student s2 = new Student("Suresh", null, 3, null);

        System.out.println(s1.equals(s2));
        System.out.println(s1.hashCode());
        System.out.println(
                MoreObjects.toStringHelper(s1)
                        .add("Name",s1.getFirstName()+" " + s1.getLastName())
                        .add("Class", s1.getClassName())
                        .add("Roll No", s1.getRollNo()));
    }
}

class Student {
    private String firstName;
    private String lastName;
    private int rollNo;
    private String className;

    public Student(String firstName, String lastName, int rollNo, String className){
        this.firstName = firstName;
        this.lastName = lastName;
        this.rollNo = rollNo;
        this.className = className;
    }

    @Override
    public boolean equals(Object object){
        if(!(object instanceof Student) || object == null){
            return false;
        }
        Student student = (Student)object;
        // no need to handle null here
        // Objects.equal("test", "test") == true
        // Objects.equal("test", null) == false
        // Objects.equal(null, "test") == false
        // Objects.equal(null, null) == true
        return Objects.equal(firstName, student.firstName) // first name can be null
                && Objects.equal(lastName, student.lastName) // last name can be null
                && Objects.equal(rollNo, student.rollNo)
                && Objects.equal(className, student.className);// class name can be null
    }

    @Override
    public int hashCode(){
        //no need to compute hashCode by self
        return Objects.hashCode(className,rollNo);
    }
    public String getFirstName() {
        return firstName;
    }
    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }
    public String getLastName() {
        return lastName;
    }
    public void setLastName(String lastName) {
        this.lastName = lastName;
    }
    public int getRollNo() {
        return rollNo;
    }
    public void setRollNo(int rollNo) {
        this.rollNo = rollNo;
    }
    public String getClassName() {
        return className;
    }
    public void setClassName(String className) {
        this.className = className;
    }
}