Extension

```
public interface Extension
```
This interface represents an X.509 extension.
Extensions provide a means of associating additional attributes with users or public keys and for managing a certification hierarchy. The extension format also allows communities to define private extensions to carry information unique to those communities.
Each extension contains an object identifier, a criticality setting indicating whether it is a critical or a non-critical extension, and and an ASN.1 DER-encoded value. Its ASN.1 definition is:
```
     Extension ::= SEQUENCE {
         extnId        OBJECT IDENTIFIER,
         critical      BOOLEAN DEFAULT FALSE,
         extnValue     OCTET STRING
                 -- contains a DER encoding of a value
                 -- of the type registered for use with
                 -- the extnId object identifier value
     }

 
```
This interface is designed to provide access to a single extension, unlike X509Extension which is more suitable for accessing a set of extensions.
Since:

1.7

Method Summary

All Methods Instance Methods Abstract Methods
Modifier and Type	Method	Description
`void`	`encode(OutputStream out)`	Generates the extension's DER encoding and writes it to the output stream.
`String`	`getId()`	Gets the extensions's object identifier.
`byte[]`	`getValue()`	Gets the extensions's DER-encoded value.
`boolean`	`isCritical()`	Gets the extension's criticality setting.

- Method Detail
  - getId
```
String getId()
```
    Gets the extensions's object identifier.
    
    Returns:
    
    the object identifier as a String
  - isCritical
```
boolean isCritical()
```
    Gets the extension's criticality setting.
    
    Returns:
    
    true if this is a critical extension.
  - getValue
```
byte[] getValue()
```
    Gets the extensions's DER-encoded value. Note, this is the bytes that are encoded as an OCTET STRING. It does not include the OCTET STRING tag and length.
    
    Returns:
    
    a copy of the extension's value, or null if no extension value is present.
  - encode
```
void encode(OutputStream out)
     throws IOException
```
    Generates the extension's DER encoding and writes it to the output stream.
    
    Parameters:
    
    out - the output stream
    
    Throws:
    
    IOException - on encoding or output error.
    
    NullPointerException - if out is null.