Java Field get()用法及代碼示例

java.lang.reflect.Field的get()方法用於獲取字段對象的值。如果Field具有原始類型，則該字段的值將自動包裝在一個對象中。如果字段是靜態字段，則obj的參數將被忽略；它可以為null，否則，基礎字段是實例字段。如果指定的obj參數為null，則此方法將引發NullPointerException；如果指定的對象不是聲明基礎字段的類或接口的實例，則此方法將引發IllegalArgumentException。如果該字段以obj類型隱藏，則該字段的值將根據前麵的規則獲得。

用法:

public double get(Object obj)
             throws IllegalArgumentException,
                    IllegalAccessException

參數：此方法接受單個參數obj，它是要從中提取字段值的對象。

返回值：此方法返回對象obj中表示字段的值；基本值在返回之前被包裝在適當的對象中。

異常：此方法引發以下異常：

IllegalAccessException–如果此Field對象正在實施Java語言訪問控製，並且基礎字段不可訪問。
IllegalArgumentException–如果指定的對象不是聲明基礎字段(或其子類或實現者)的類或接口的實例。
NullPointerException –如果指定的對象為null，並且該字段是實例字段。
ExceptionInInitializerError–如果此方法引發的初始化失敗。

以下示例程序旨在說明get()方法：
示例1:

// Java program to demonstrate get() method 
  
import java.lang.reflect.Field; 
  
public class GFG { 
  
    public static void main(String[] args) 
        throws Exception 
    { 
  
        // Create the User class object 
        User user = new User(); 
  
        // Get the all field objects of User class 
        Field[] fields = User.class.getFields(); 
  
        for (int i = 0; i < fields.length; i++) { 
  
            // get value of the fields 
            Object value = fields[i].get(user); 
  
            // print result 
            System.out.println("Value of Field "
                               + fields[i].getName() 
                               + " is " + value); 
        } 
    } 
} 
  
// sample User class 
class User { 
  
    // static double values 
    public static double Marks = 34.13; 
    public static int Fees = 34199; 
    public static String name = "Aman"; 
  
    public static double getMarks() 
    { 
        return Marks; 
    } 
  
    public static void setMarks(double marks) 
    { 
        Marks = marks; 
    } 
  
    public static int getFees() 
    { 
        return Fees; 
    } 
  
    public static void setFees(int fees) 
    { 
        Fees = fees; 
    } 
  
    public static String getName() 
    { 
        return name; 
    } 
  
    public static void setName(String name) 
    { 
        User.name = name; 
    } 
}

輸出：

Value of Field Marks is 34.13
Value of Field Fees is 34199
Value of Field name is Aman

示例2:

// Java program to demonstrate get() method 
  
import java.lang.reflect.Field; 
  
public class GFG { 
  
    public static void main(String[] args) 
        throws Exception 
    { 
  
        // Create the RealNumbers class object 
        Fields field = new Fields(); 
  
        // Get the all field objects of User class 
        Field[] fieldsOfFieldClass 
            = Fields.class.getFields(); 
  
        for (int i = 0; 
             i < fieldsOfFieldClass.length; i++) { 
  
            // get value 
            Object value = fieldsOfFieldClass[i] 
                               .get(field); 
  
            // print result 
            System.out.println( 
                "Value of Field "
                + fieldsOfFieldClass[i].getName() 
                + " is " + value); 
        } 
    } 
  
    // RealNumbers class 
    static class Fields { 
  
        // double field 
        public static final double doubleValue 
            = 9999999.34567; 
        public static final int intValue 
            = 9999999; 
        public static final float floatValue 
            = 9999999.34567f; 
        public static final
            boolean booleanValue 
            = true; 
    } 
}

輸出：

Value of Field doubleValue is 9999999.34567
Value of Field intValue is 9999999
Value of Field floatValue is 9999999.0
Value of Field booleanValue is true

參考文獻： https://docs.oracle.com/javase/8/docs/api/java/lang/reflect/Field.html#get-java.lang.Object-

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注：本文由純淨天空篩選整理自AmanSingh2210大神的英文原創作品 Field get() method in Java with Examples。非經特殊聲明，原始代碼版權歸原作者所有，本譯文未經允許或授權，請勿轉載或複製。