PostgreSQL, 158 characters

select'public static boolean isPowerOfTen(long input) {
  return
   '||string_agg(' input == 1'||repeat('0',x)||'L','
  ||')||';
}'from generate_series(0,18)x

05AB1E, 99 97 96 94 93 bytes

Code:

“‚Æ£‹ÒŒ€ˆPowerOfTen(“ ?“¢„î®) {“ ,“ «‡
   “ ?19FN0›i"  ||"?}’ î® == ’ ?N°?'L?N18Qi';,"}"?}"",

Uses CP-1252 encoding.

Java, 217 215 bytes

Golfed:

public static void a(){String s="public static boolean isPowerOfTen(long input) {\n\treturn input == 1L";for(int i=1;i<18;i++){s+="\n\t|| input == 1";for(int k=0;k<i;k++)s+="0";s+="L";}System.out.println(s+";\n}");}

Ungolfed:

public static void a(){
    String s = "public static boolean isPowerOfTen(long input) {\n\treturn input == 1L";
    for (int i = 1; i < 18; i++) {
        s += "\n\t|| input == 1";
        for (int k = 0; k<i; k++)
            s += "0";
        s += "L";
    }
    System.out.println(s + ";\n}");
  }
}

(first answer, wuhu)

Thanks!
-2 bytes: user902383

Pyth, 118 106 103 bytes

s[."
{Z-L¡JxÙÿ
LæÝ<­í?¢µb'¥ÜA«Ç}h¹äÚÏß"\nb*4dj"\n  || "ms[." uøs|ÀiÝ"*d\0\L)U19\;b\}

Try it online!

All this string hardcoding really eats a lot of bytes up, but nothing I can do about it.

Update: Saved 3 bytes by using a packed string. Thanks @user81655 for the hint!

Ruby, 125 bytes

$><<'public static boolean isPowerOfTen(long input) {
  return
   '+(0..19).map{|i|" input == #{10**i}L
  ||"}.join[0..-5]+?}

Explanation to come.

Oracle SQL 9.2, 311 bytes

SELECT REPLACE(REPLACE('public static boolean isPowerOfTen(long input) {'||CHR(10)||'  return'||c||';'||'}', 'n  ||', 'n'||CHR(10)||'   '),CHR(10)||';', ';'||CHR(10)) FROM(SELECT LEVEL l,SYS_CONNECT_BY_PATH('input == '||TO_CHAR(POWER(10,LEVEL-1))||'L'||CHR(10),'  || ')c FROM DUAL CONNECT BY LEVEL<20)WHERE l=19

Javascript, 172 157 152 150 148 bytes

p=>`public static boolean isPowerOfTen(long input) {
  return${[...Array(19)].map((x,i)=>`
  ${i?'||':' '} input == 1${'0'.repeat(i)}L`).join``};
}`

f=p=>`public static boolean isPowerOfTen(long input) {
  return${[...Array(19)].map((x,i)=>`
  ${i?'||':' '} input == 1${'0'.repeat(i)}L`).join``};
}`

document.body.innerHTML = '<pre>' + f() + '</pre>'

C, 158 155 bytes

i;main(){for(puts("public static boolean isPowerOfTen(long input) {\n  return");i<19;)printf("  %s input == 1%0.*dL%s\n",i++?"||":" ",i,0,i<18?"":";\n}");}

Try it online here.

C# (CSI) 181 180 179 byte

string i=" input == 1",e="public static bool";Console.Write(e+@"ean isPowerOfTen(long input) {
  return
   "+i+string.Join(@"L
  ||"+i,e.Select((_,x)=>new string('0',x)))+@"L;
}")

There is only one little trick involved. The straight forward way to write this would be:

string.Join("L\n  || input == 1",Enumerable.Range(0,18).Select(x=>new string('0',x)))

by using the string with the first 18 characters of the text which I need anyways I can get rid off the lengthy Enumerable.Range. This works because string implements IEnumerable and there is a version of Select that hands the item (not needed) and the index which we want to the lambda function.

Perl 5 - 130 141

@s=map{'input == 1'.0 x$_."L\n  ||"}0..18;$s[$#s]=~s/\n  \|\|/;\n}/g;print"public static boolean isPowerOfTen(long input){\n  return\n    @s"

EDIT: fixed to have exact indentation

jq, 123 characters

(121 characters code + 2 characters command line option.)

"public static boolean isPowerOfTen(long input) {
  return
   \([range(19)|" input == 1\("0"*.//"")L"]|join("
  ||"));
}"

Sample run:

bash-4.3$ jq -nr '"public static boolean isPowerOfTen(long input) {
>   return
>    \([range(19)|" input == 1\("0"*.//"")L"]|join("
>   ||"));
> }"'
public static boolean isPowerOfTen(long input) {
  return
    input == 1L
  || input == 10L
  || input == 100L
  || input == 1000L
  || input == 10000L
  || input == 100000L
  || input == 1000000L
  || input == 10000000L
  || input == 100000000L
  || input == 1000000000L
  || input == 10000000000L
  || input == 100000000000L
  || input == 1000000000000L
  || input == 10000000000000L
  || input == 100000000000000L
  || input == 1000000000000000L
  || input == 10000000000000000L
  || input == 100000000000000000L
  || input == 1000000000000000000L;
}

Python (3.5) 137 bytes

print("public static boolean isPowerOfTen(long input) {\n  return\n   ",'\n  || '.join("input == 1"+"0"*i+"L"for i in range(19))+";\n}")

PowerShell, 120 bytes

'public static boolean isPowerOfTen(long input) {'
'  return'
"   $((0..18|%{" input == 1"+"0"*$_})-join"L`n  ||")L;`n}"

The first two lines are simply string literals, which are output as-is.

The third line starts with three spaces, and ends with L;`n}" to finish off the last couple bytes. The middle bit inside the script block $(...) is constructed by for-looping % from 0 to 18 and each iteration constructing a string that starts with input == 1 concatenated with the corresponding number of zeros. This will spit out an array of strings. We then -join each element of the array with L`n || to achieve the newline-pipes. That big string is the output of the script block, which gets inserted automatically into the middle and output.

PS C:\Tools\Scripts\golfing> .\go-generate-some-java.ps1
public static boolean isPowerOfTen(long input) {
  return
    input == 1L
  || input == 10L
  || input == 100L
  || input == 1000L
  || input == 10000L
  || input == 100000L
  || input == 1000000L
  || input == 10000000L
  || input == 100000000L
  || input == 1000000000L
  || input == 10000000000L
  || input == 100000000000L
  || input == 1000000000000L
  || input == 10000000000000L
  || input == 100000000000000L
  || input == 1000000000000000L
  || input == 10000000000000000L
  || input == 100000000000000000L
  || input == 1000000000000000000L;
}

Jelly, 75 bytes

(These are bytes in Jelly's custom codepage.)

0r18⁵*;@€⁶j“¢œḤḅg^NrÞḢ⁷ẉ»“⁵®UẆƓḃÐL⁴ṖịṛFþẈ¹9}¶ ƁḋȮ¦sẒẆd€Ḟɼ¿ỌṀP^µ\f@»;;“L;¶}”

Try it here.

Explanation

0r18      Range [0..18]
⁵*        Take the 10^ of each number
;@€⁶      Prepend a space to each number
j“...»    Join by compressed string "L\n  || input =="
“...»;    Prepend compressed string "public static ... =="
;“L;¶}”   Append "L;\n}"

T-SQL 289 bytes

WITH A AS(SELECT CAST('1' AS VARCHAR(20)) N UNION ALL SELECT CAST(CONCAT(N,'0') AS VARCHAR(20))FROM A WHERE LEN(N)<20)SELECT 'public static boolean isPowerOfTen(long input){return '+STUFF((SELECT '|| input=='+N+'L ' FROM A FOR XML PATH(''),TYPE).value('.', 'VARCHAR(MAX)'), 1, 3, '' )+';}'

Javascript 175 bytes

Let's do this regularly

var s = "public static boolean isPowerOfTen(long input) {\n\treturn\n\t\tinput == 1";
for (var i = 1; i < 20; i++) {
    s += "\n\t|| input == 1";
    for (var j = 0; j < i; j++) {
        s += "0";
    }
    s += "L" ;
}
s += ";\n}";
alert(s);

Pretty small. Now, some javascript magic, like no semicolons needed, or no var's, etc.:

k="input == 1"
s="public static boolean isPowerOfTen(long input) {\n\treturn\n\t\t"+k+"L"
for(i=1;i<20;i++){s+="\n\t|| "+k
for(j=0;j<i;j++)s+="0";s+="L"}s+=";\n}"
alert(s)

Java 215

class A{public static void main(String[]a){String s="public static boolean isPowerOfTen(long input) {\n  return\n   ";for(long i=1;i>0;i*=10)s+=" input == "+i+"L\n  ||";System.out.print(s.substring(0,515)+";\n}");}}

or if

you write a shorter program or function that outputs this function for you

can be interpreted to allow for writing a Java method that returns a String, then this can be golfed down to 171:

String a(){String s="public static boolean isPowerOfTen(long input) {\n  return\n   ";for(long i=1;i>0;i*=10)s+=" input == "+i+"L\n  ||";return s.substring(0,515)+";\n}";}

ES6, 139 bytes

_=>"0".repeat(19).replace(/./g,`
 || input == 1$\`L`).replace(`
 ||`,`public static boolean isPowerOfTen(long input) {
  return\n  `)+`;
}`

I do so love these triangle generation questions.

Java, 210 / 166

Score is depending on whether returning the input from a function meets the definition of 'output'.

Console output (210):

class A{public static void main(String[]a){String a=" input == 1",t="L\n  ||"+a,s="public static boolean isPowerOfTen(long input) {\n  return\n   "+a;for(int i=0;++i<19;)s+=t+="0";System.out.print(s+"L;\n}");}}

String return (166):

String a(){String a=" input == 1",t="L\n  ||"+a,s="public static boolean isPowerOfTen(long input) {\n  return\n   "+a;for(int i=0;++i<19;)s+=t+="0";return s+"L;\n}";}

Legible version:

String a() {
    String a=" input == 1", t = "L\n  ||"+a,
        s = "public static boolean isPowerOfTen(long input) {\n  return\n   "+a;
    for (int i = 0; ++i < 19;)
        s += t += "0";
    return s + "L;\n}";
}