chore: Create helper method to extract the type field

Author: lqiu96

oauth2_http/java/com/google/auth/oauth2/ExternalAccountAuthorizedUserCredentials.java

@@ -162,10 +162,7 @@ public static ExternalAccountAuthorizedUserCredentials fromStream(
       InputStream credentialsStream, HttpTransportFactory transportFactory) throws IOException {
     Preconditions.checkNotNull(transportFactory);
     GenericJson fileContents = parseJsonInputStream(credentialsStream);
-    String fileType = (String) fileContents.get("type");
-    if (fileType == null) {
-      throw new IOException("Error reading credentials from stream, 'type' field not specified.");
-    }
+    String fileType = extractFromJson(fileContents, "type");
     if (fileType.equals(
         GoogleCredentialsInfo.EXTERNAL_ACCOUNT_AUTHORIZED_USER_CREDENTIALS.getFileType())) {
       try {

oauth2_http/java/com/google/auth/oauth2/ExternalAccountCredentials.java

@@ -388,10 +388,7 @@ public static ExternalAccountCredentials fromStream(
       InputStream credentialsStream, HttpTransportFactory transportFactory) throws IOException {
     Preconditions.checkNotNull(transportFactory);
     GenericJson fileContents = parseJsonInputStream(credentialsStream);
-    String fileType = (String) fileContents.get("type");
-    if (fileType == null) {
-      throw new IOException("Error reading credentials from stream, 'type' field not specified.");
-    }
+    String fileType = extractFromJson(fileContents, "type");
     if (fileType.equals(GoogleCredentialsInfo.EXTERNAL_ACCOUNT_CREDENTIALS.getFileType())) {
       try {
         return fromJson(fileContents, transportFactory);

oauth2_http/java/com/google/auth/oauth2/GdchCredentials.java

@@ -140,10 +140,7 @@ public static GdchCredentials fromStream(
       InputStream credentialsStream, HttpTransportFactory transportFactory) throws IOException {
     Preconditions.checkNotNull(transportFactory);
     GenericJson fileContents = parseJsonInputStream(credentialsStream);
-    String fileType = (String) fileContents.get("type");
-    if (fileType == null) {
-      throw new IOException("Error reading credentials from stream, 'type' field not specified.");
-    }
+    String fileType = extractFromJson(fileContents, "type");
     if (fileType.equals(GoogleCredentialsInfo.GDCH_CREDENTIALS.getFileType())) {
       return fromJson(fileContents, transportFactory);
     }

oauth2_http/java/com/google/auth/oauth2/GoogleCredentials.java

@@ -244,6 +244,19 @@ static GenericJson parseJsonInputStream(InputStream credentialsStream) throws IO
     return parser.parseAndClose(credentialsStream, StandardCharsets.UTF_8, GenericJson.class);
   }
 
+  /**
+   * Internal helper method to try and extract a field from the json stream and throw an exception
+   * if it doesn't exist.
+   */
+  static String extractFromJson(Map<String, Object> json, String field) throws IOException {
+    String fileType = (String) json.get(field);
+    if (fileType == null) {
+      throw new IOException(
+          "Error reading credentials from stream, '" + field + "' field not specified.");
+    }
+    return fileType;
+  }
+
   /**
    * This method is obsolete because of a potential security risk. Use the credential specific load
    * method instead

oauth2_http/java/com/google/auth/oauth2/ImpersonatedCredentials.java

@@ -403,10 +403,7 @@ public static ImpersonatedCredentials fromStream(
       InputStream credentialsStream, HttpTransportFactory transportFactory) throws IOException {
     Preconditions.checkNotNull(transportFactory);
     GenericJson fileContents = parseJsonInputStream(credentialsStream);
-    String fileType = (String) fileContents.get("type");
-    if (fileType == null) {
-      throw new IOException("Error reading credentials from stream, 'type' field not specified.");
-    }
+    String fileType = extractFromJson(fileContents, "type");
     if (fileType.equals(GoogleCredentialsInfo.IMPERSONATED_CREDENTIALS.getFileType())) {
       return fromJson(fileContents, transportFactory);
     }

oauth2_http/java/com/google/auth/oauth2/ServiceAccountCredentials.java

@@ -488,10 +488,7 @@ public static ServiceAccountCredentials fromStream(
       InputStream credentialsStream, HttpTransportFactory transportFactory) throws IOException {
     Preconditions.checkNotNull(transportFactory);
     GenericJson fileContents = parseJsonInputStream(credentialsStream);
-    String fileType = (String) fileContents.get("type");
-    if (fileType == null) {
-      throw new IOException("Error reading credentials from stream, 'type' field not specified.");
-    }
+    String fileType = extractFromJson(fileContents, "type");
     if (fileType.equals(GoogleCredentialsInfo.SERVICE_ACCOUNT_CREDENTIALS.getFileType())) {
       return fromJson(fileContents, transportFactory);
     }

oauth2_http/java/com/google/auth/oauth2/UserCredentials.java

@@ -174,10 +174,7 @@ public static UserCredentials fromStream(
       InputStream credentialsStream, HttpTransportFactory transportFactory) throws IOException {
     Preconditions.checkNotNull(transportFactory);
     GenericJson fileContents = parseJsonInputStream(credentialsStream);
-    String fileType = (String) fileContents.get("type");
-    if (fileType == null) {
-      throw new IOException("Error reading credentials from stream, 'type' field not specified.");
-    }
+    String fileType = extractFromJson(fileContents, "type");
     if (fileType.equals(GoogleCredentialsInfo.USER_CREDENTIALS.getFileType())) {
       return fromJson(fileContents, transportFactory);
     }